lo. 


BW6CENCES  LIBRARY 


Agric.-  Genetics 


v, 


—    xos^e  V» 


Contents 

Studies  on  chromosomes 

I.  The  behavior  of  the  idiochromosomes  in 
Hemiptera. 

II.  The  paired  Mi  crochromo  somes  ,  idiochromo- 
somes and  heterotropic,  chromosomes  in 
Hemiptera. 

III.  The  sexual  differences  of  the  Chromosome- 
groups  in  Hempitera,  with  some  considera- 
tions of  the  determination  and  inheritance 
of  sex. 

IV.  The  "accessory"  chromosome  in  Syromastes 
and  Pyrrochoris  with  a  comparative  review 
of  the  types  of  sexual  differences  of  the 
chromosome  groups. 

V.  The  chromosomes  of  Metapodius,  a  contribu- 
tion to  the  hypothesis  of  the  genetic  con- 

tinuity of  chromosomes. 

VI.  A  new  type  of  chromosome  combination  in 
inoteipha.se. 


VII.  A  review  of  the  chromosomes  of  Nezara; 
with  some  more  general  considerations. 

VIII.  Observations  on  the  maturation-  phenome- 
na in  certain  Hempitera  and  other  fonns 
with  considerations  on  synapsis  and  re- 
duction. 


967569 


E 


With  the  compliments  of 

EDM,  B,  WILSON, 

COLUMBIA  UNIVERSITY,  NEW  YORK, 


STUDIES  ON  CHROMOSOMES 

THE    BEHAVIOR    OF    THE    IDIOCHROMOSOMES 
IN    HEMIPTERA 


By 
EDMUND   B.  WILSON 


RETURN  TO 
DIVISION  OF  GENETICS 
HILGARD  HALL     ,   . 


REPRINTED   FROM 

THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY 

Volume   II 
No.  3 


BALTIMORE,  MD.,  U.  S.  A. 

August,  1905 


STUDIES  ON  CHROMOSOMES. 

I.     THE    BEHAVIOR    OF    THE    IDIOCHROMOSO.MES    IN 

HEMIPTERA.1 


EDMUND    B.  WILSON. 

WITH  7  FIGURES. 

In  studying  the  spermatocyte-divisions  in  Lygaeus  turcicus  and 
Coenus  delius,  and  afterward  in  several  other  genera  of  Hemip- 
tera,  my  attention  was  directed  to  the  fact  that  the  number  of 
chromosomes  appeared  to  vary,  polar  views  of  the  equatorial 
plate  showing  sometimes  seven  chromosomes,"  sometimes  eight 
(cf.  Figs.  iby  it,  2a,  2i,  3</,  3/,  etc.).  Montgomery, iritis  fe^nsive 
comparative  paper  of  1901,  describes  and  figures  a  similar'  Varia- 
tion in  a  number  of  cases,  including  Coenus  delius  and  Euschistus 
tristigmus  ('01,  I,  pp.  161,  166),  and  in  the  latter  case  considered 
it  as  a  result  of  variations  in  the  synapsis  of  the  two  "chromatin 
nucleoli"  which  he  supposed  might  either  conjugate  to  form  a 
bivalent  body  before  the  first  division  (in  which  case  this  division 

'This  paper  is  based  on  a  study  of  some  very  fine  series  of  sections  of  the  testes  of  certain  Hemiptera, 
prepared  six  or  eight  years  ago  by  Dr.  F.  C.  Paulmier,  in  connection  with  his  valuable  paper  on  the 
spermatogenesis  of  Anasa  tristis  ('99).  Part  of  the  original  Anasa  sections,  with  a  number  of  series  of 
the  testes  of  some  other  insects,  were  given  to  the  cytological  cabinet  of  the  Columbia  laboratory  at  that 
time;  some  of  the  best  of  the  remainder  were  subsequently  loaned  to  Mr.Sutton,  and  others  to  Dr.  Dublin, 
for  comparison  with  their  work  on  the  spermatogenesis  of  other  forms.  Certain  inconsistencies  in  the 
literature  relating  to  the  accessory  chromosome  and  the  microchromosomes  or  "chromatin-nucleoli " 
led  me  to  re-examine  the  preparations  of  Anasa  and  some  of  the  other  genera,  which  yielded  some  new 
and  interesting  conclusions  in  the  case  of  Anasa,  and  also  of  Alydus,  Lygaeus  and  Ccenus.  Dr.  Paulmier 
being  preoccupied  with  other  lines  of  work  did  not  find  it  practicable  again  to  take  up  his  cytological 
studies,  and  he  was  generous  enough  to  give  me,  for  the  laboratory,  his  entire  set  of  preparations,  com- 
prising, in  addition  to  the  slides  already  given  or  loaned,  serial  sections  of  more  than  a  hundred  testes 
representing  upward  of  twenty  genera  of  Hemiptera  and  other  insects.  A  typical  series  of  the  Hemip- 
tera from  which  these  testes  were  taken  had  been  identified  by  the  eminent  specialist,  Mr.  P.  R.  Uhler. 
Much  of  this  material  is  admirably  fixed,  sectioned  and  stained,  and  the  best  preparations  are  a  model  of 
technical  excellence,  showing  especially  the  chromosomes  of  the  spermatogonial  and  spermatocyte 
divisions  with  a  clearness  and  brilliancy  comparable  with  that  of  the  best  Ascaris  preparations.  The 


372  Edmund  B.  Wilson. 

was  described  as  showing  but  seven  chromosomes)  or  might  remain 
separate  during  this  division  (in  which  case  eight  separate  chro- 
mosomes appear).  I  soon  found,  however,  that  in  Lygaeus  and 
Coenus  whole  cysts  differed  in  this  respect,  all  of  the  cells  of  a  given 
cyst  constantly  showing  one  number  or  the  other.  With  this  is 
correlated  the  fact  that  in  the  anaphases  of  the  second  division 
no  accessory  chromosome  in  the  usual  sense  of  the  term  is  present, 
all  the  spermatid-nuclei  receiving  the  same  number  of  chromo- 
somes, namely,  seven,  which  is  half  the  spermatogonial  number  in 
both  species.  Further  study  conclusively  showed  that  in  both  of 
the  species  the  cells  with  eight  chromosomes  were  primary  sper- 
matocytes  undergoing  the  first  maturation-division,  while  those 
with  seven  were  the  secondary  spermatocytes  undergoing  the 
second  division.  Of  this  fact  no  doubt  can  exist,  since  the  second- 
ary spermatocytes  are  much  smaller  than  the  primary  ones,  the 
spindles  are  shorter,  the  chromosomes  only  half  as  large,  the  meta- 
pha*sej-£|rures<*ajre  cjftQnffound  in  the  same  cysts  with  the  character- 
istic^  late  ^anapjiases  and^telophases,  and  all  the  stages  of  both 
dfj^5ifcjfe;^r^'^ijff4Mti.^t)undance.  Though  a  great  number  of 
division-figures  have  been  examined,  I  have  never  seen  seven 
chromosomes  in  the  first  division  in  any  of  the  forms  examined; 
and  though  I  will  not  deny  that  Montgomery  may  be  correct  in 
the  statement  that  such  forms  occur,  I  believe  he  was  misled  on 


most  successful  preparations  are  from  material  fixed  with  strong  Flemming's  fluid,  and  stained  with  iron 
haematoxylin  followed  by  long  extraction,  in  some  cases  followed  also  by  counter  staining  with  Congo 
red  or  orange  G.  These  show  the  cytoplasm  completely  decolorized,  the  chromosomes  intensely  black, 
and  with  outlines  of  such  regularity  and  sharpness  that  the  most  careful  camera  drawings  give  the  appear- 
ance of  being  schematized.  A  few  very  fine  series  were  stained  with  Zwaardemaker's  saffranin  (which 
gives  a  splendid  transparent  stain).  Many  others  were  fastened  to  the  slide  unstained,  and  some  of 
these  I  have  stained  with  saffranin  and  gentian  violet  (the  method  recommended  by  Montgomery) 
which  have  given  very  valuable  control  results,  especially  in  regard  to  the  accessory  chromosome  and 
plasmosome  which  in  the  earlier  growth  stages  are  not  well  differentiated  by  the  haematoxylin.  I  am 
much  indebted  to  Dr.  Paulmier's  generosity  in  placing  at  my  disposal  this  valuable  material,  to  which  I 
have  since  added  many  new  preparations  of  my  own. 

In  a  subsequent  paper  I  shall  describe  the  results  of  a  re-examination  of  some  of  the  maturation 
phenomena  in  Anasa  and  Alydus,  in  both  of  which  there  is  demonstrative  evidence  that  the  accessory 
chromosome  is  not  the  small  central  chromosomeor  microchromosome  ("  chromatin-nucleolus"  of  Mont- 
gomery), as  Paulmier  supposed  in  the  case  of  Anasa,  but  the  odd  or  peripheral  one,  precisely  as  Gross 
('04)  has  recently  described  in  Syromastes.  While  looking  over  some  of  the  other  species  for  the  sake  of 
comparison  my  attention  was  directed,  first  in  the  spermatogenesis  of  Lygaeus  turcicus  and  Ccenus 
delius  and  afterward  in  that  of  Euschistus,  Podisus  and  other  forms,  to  the  phenomena  which  form 
the  subject  of  the  present  paper. 


Studies  on  Chromosomes.  373 

this  point  by  failing  in  some  cases  to  distinguish  between  the  two 
divisions. 

On  tracing  out  the  history  of  the  two  divisions  step  by  step, 
decisive  proof  was  obtained  that  the  apparent  reduction  in  number 
is  brought  about  in  the  period  immediately  following  the  final 
anaphase  of  the  first  division  (which  coincides  with  the  earliest 
prophase  of  the  second  division)  by  a  conjugation  of  two  unequal 
chromosomes  that  occupy  the  center  of  the  equatorial  plate  in  the 
first  division  and  evidently  correspond  to  some  of  the  forms 
designated  by  Montgomery  as  "chromatin-nucleoli."  This  pro- 
cess can  be  determined  with  certainty,  owing  to  the  fact  that  in  all 
of  the  species,  with  a  single  exception,  one  of  the  two  conjugating 
chromosomes  is  much  smaller  than  the  others,  while  in  Lygaeus 
both  are  much  smaller,  and  they  are  very  unequal  in  size.  The 
central  dyad  of  the  second  division  is  therefore  asymmetrical,  one 
of  its  constituents  being  in  Lygaeus  not  less  than  five  or  six,  and 
in  Coenus  not  less  than  two  or  three,  times  the  bulk  of  the  other. 
The  two  unequal  constituents  of  this  dyad  are  then  immediately 
separated  again  in  the  ensuing  division  in  such  a  manner  that  in 
both  species  one  half  the  spermatids  receive  the  smaller,  one  half 
the  larger,  moiety  of  the  central  chromosome  (or  dyad}  of  the  second 
division.  An  essentially  similar  process  was  ultimately,  found  to 
occur  in  Euschistus  fissilis,  in  another  undetermined  species  of 
the  same  genus,  in  Brochymena,  Nezara,  Podisus  and  Tricho- 
pepla.  The  first  four  of  these  show  the  same  chromosome- 
numbers  as  in  Lygaeus  and  Coenus.  In  Podisus  the  number  is  in 
each  division  one  more  than  in  the  corresponding  divisions  of  the 
other  genera  (/.  e.,  respectively  9  and  8  instead  of  8  and  7,  while 
the  spermatogonial  number  is  16  instead  of  14). 1  Nezara  differs 
from  the  other  genera  in  the  fact,  which  is  of  importance  for  a 
comparison  with  such  forms  as  Anasa  or  Alydus,  that  the  two 
chromosomes  which  undergo  conjugation  after  the  first  division 
are  of  equal  size;  so  that  in  this  form  the  two  classes  of  spermatids 
are  indistinguishable  by  the  eye.  Since  the  eight  species  I  have 


JIn  several  of  these  cases  the  numbers  do  not  agree  with  those  given  by  Montgomery  ('01,  i).  I 
believe  this  observer  to  have  been  misled  by  the  fact,  which  he  also  observed  in  some  cases,  that  the  first 
division  shows  one  more  than  half  the  spermatogonial  number  of  chromosomes;  and  it  is  easy  to  mistake 
the  latter  number  owing  to  the  fact  that  the  larger  spermatogonial  chromosomes  often  show  a  more  or 
less  marked  constriction  in  the  middle.  Slightly  oblique  views  of  the  late  metaphase,  when  the  chromo- 
somes are  double,  may  also  readily  give  an  erroneous  result. 


374  Edmund  B.  Wilson. 

examined  represent  two  different  families  of  Hemiptera  (Penta- 
tomidae  and  Lygaeidae)  the  idiochromosomes  will  probably  be 
found  to  be  of  wide  occurrence  in  the  group.1  The  only  other  case 
known  to  me  in  any  higher  plant  or  animal  of  the  unequal  division 
of  a  chromosome  (or  chromatin-body)  in  karyokinesis  occurs  in 
Tingis  clavata,  regarding  which  Montgomery  states  that  one  of 
the  chromosomes  of  the  first  division  "very  frequently  is  seen  to  be 
characterized  in  having  its  two  components  of  very  unequal  vol- 
ume" ('01,  2,  p.  262).  This  author  also  observed  a  considerable 
number  of  cases  in  which  the  "chromatin  nucleoli"  are  unequal 
in  the  rest  stage  of  the  spermatogoma,  and  he  describes  some  forms 
in  which  a  similar  condition  appears  in  the  growth-period  of  the 
spermatocytes  (e.  g.,  in  Trichopepla,  Peribalus  and  Euschistus 
tristigmus).  In  the  last-named  species  he  found  that  a  separation 
of  the  two  unequal  "chromatin  nucleoli"  takes  place  in  the  second 
mitosis  ('01,  I,  pp.  161,  162),  but  expressly  states  that  they  are  not 
joined  together  in  the  equatorial  plate  (op.  cit.,  p.  162).  It  is  evi- 
dent from  Montgomery's  brief  description  that  this  phenomenon 
is  similar  to,  and  probably  identical  with,  the  one  that  forms  the 
subject  of  this. paper. 

TERMINOLOGY. 

Since  confusion  may  readily  arise  in  the  terminology,  I  wish  to 
define  clearly  the  terms  that  will  be  employed  throughout  this 
paper  and  its  successors.  I  shall  apply  the  term  "chromosome" 
to  each  coherent  chromatin-mass,  whatever  be  its  form,  mode  of 
origin  or  valence,  which  as  such  enters  the  equatorial  plate.  In 
the  case  of  compound  or  plurivalent  chromosomes  ("tetrads"  or 
"dyads")  McClung's  term  "chromatid"  may  conveniently  be 
applied  to  each  of  their  univalent  constituents.  I  may  call  atten- 
tion, in  connection  with  this,  to  the  fact  that  the  valence  of  chro- 
mosomes cannot  be  determined  by  mere  inspection  of  their  form. 
In  many  Hemiptera,  for  example,  the  chromosomes  of  the  first 
maturation-division  frequently  show  a  dyad-like  or  dumb-bell 
shape  (typically  the  case,  for  example,  in  Euschistus,  Lygaeus  or 
Coenus)  even  though  in  earlier  stages  they  are  plainly  quadripar- 

'Since  writing  the  above  I  have  found  the  idiochromosomes  in  several  additional  genera.  In 
Mineus  they  are  only  slightly  unequal,  in  Murgantia  nearly  as  unequal  as  in  Lygaeus.  Nezara, 
Mineus,  Brochymena,  Euschistus,  Murgantia  and  Lygaeus  thus  show  a  progressively  graded  series 
of  stages  in  the  size-differentiation  of  this  peculiar  pair  of  chromosomes. 


Studies  on  Chromosomes.  375 

tite;  and  such  dyad-like  forms,  agreeing  both  in  mode  of  origin  and 
in  fate  with  actual  tetrads,  may  occur  in  the  same  equatorial  plate 
with  obviously  quadripartite  forms  (cf.  Fig.  2e}.  Conversely  it 
will  be  shown  beyond  that  bivalent  and  univalent  chromosomes 
occurring  in  the  same  equatorial  plate  may  exactly  agree  in  form, 
though  having  a  wholly  different  mode  of  origin. 

The  purely  descriptive  term  "idiochromosomes"  (peculiar 
or  distinctive  chromosomes)  will  be  applied  to  the  two  chromo- 
somes, usually  unequal  in  size,  which,  as  stated  above,  undergo  a 
very  late  conjugation  and  subsequent  asymmetrical  distribution  to 
the  spermatid-nuclei.  These  bodies,  as  already  stated,  are  iden- 
tical with  some  of  those  to  which  Montgomery  ('01,  '04)  has 
applied  the  term  "chromatin  nucleoli."  This  use  of  the  latter 
term  is,  however,  undesirable,  since  the  accessory  chromosome 
also  appears  in  the  growth-period  (of  Orthoptera  and  some 
Hemiptera)  in  the  form  of  a  chromatin-nucleolus.  I  shall,  there- 
fore, employ  the  latter  term  in  a  broader  sense  to  designate  any 
compact  deeply  staining  chromatin-mass,  present  in  the  resting 
nucleus,  which  afterward  contributes  to  the  formation  of  the 
chromosomes.  When,  as  in  case  of  the  accessory  chromosome, 
or  the  idiochromosomes,  such  a  chromatin-nucleolus  represents 
a  single  chromosome  or  pair  of  chromosomes  it  may  conveniently 
be  called  a  "chromosome-nucleolus";  but  I  think  this  term  should 
be  restricted  to  the  resting  nuclei  and  cannot  appropriately  be 
applied  to  the  corresponding  chromosome  of  the  division-stage. 
Especially  large  or  small  chromosomes  may  be  designated  as 
"macrochromosomes"  or  "microchromosomes,"  irrespective  of 
their  behavior. 

DESCRIPTIVE. 

In  the  following  account  Lygaeus  and  Coenus  will  be  taken  as 
types,  a  brief  comparison  of  the  other  forms  being  added.  Some 
of  the  latter — especially  Brochymena  and  Nezara — present  features 
of  peculiar  interest  which  I  hope  to  make  the  subject  of  a  special 
study  hereafter. 

I.     The  Maturation  Divisions. 

Lygaeus  and  Coenus  show  an  extremely  close  agreement  in  the 
general  history  of  the  chromosome-group,  and  especially  in  the 
behavior  of  the  idiochromosomes;  though  the  earlier  history  of 


376  Edmund  B.  Wilson. 

these  bodies  shows  a  more  primitive  condition  in  the  former  genus. 
I  have  followed  their  behavior  in  the  early  stages  less  completely 
in  Euschistus  and  Podisus,  but  their  behavior  during  the  matura- 
tion-divisions in  these  forms  is  closely  similar  to  that  of  the  others 
and  leads  to  an  exactly  similar  result.  Lygaeus  is  in  some  respects 
the  most  favorable  of  all  these  species  owing  to  the  remarkable 
disparity  in  size  between  the  idiochromosomes,  and  to  the  fact 
that  both  are  so  much  smaller  than  the  other  chromosomes  as  to 
admit  of  their  immediate  identification  at  every  period. 

In  all  the  species,  the  chromosomes  show  distinct  and  constant 
size-differences.     A   largest    chromosome    or    macrochromosome 

O 

may  be  distinguished  in  all,  and  in  most  cases  a  second  largest; 
and  in  all,  the  small  idiochromosome  is  the  smallest  of  the  group 
and  typically  lies  near  the  center  of  the  equatorial  plate.  (Figs. 
ib,  2<r,  2d,  3#,  d,  /,  etc.)  It  is  difficult  to  be  sure  of  the  size-differ- 
ences in  case  of  the  other  chromosomes,  owing  to  variations  in 
form  and  position,which  produce  various  degrees  of  foreshortening. 
In  all  the  forms,  with  the  exception  of  Nezara,  the  larger  chromo- 
somes of  the  first  division  are  typically  arranged  in  an  irregular 
ring  within  which  lie  the  two  idiochromosomes,  side  by  side,  but 
always  quite  separate  (Figs.  l&,  2a,  30,  d,  /,  etc.).  This  grouping 
is  apparently  invariable  in  Lygaeus,  but  in  Coenus  and  Euschistus 
the  larger  idiochromosome  frequently  lies  in  the  outer  ring  (Figs. 
2b,  3*).  In  Lygaeus  the  two  idiochromosomes,  within  the  ring, 
are  always  much  smaller  than  any  of  the  outer  ones,  and  the 
smaller  is  so  minute  that  at  first  sight  I  mistook  it  for  a  centro- 
some.1  In  Coenus  both  the  idiochromosomes  are  relatively 
larger  than  in  Lygaeus,  and  their  inequality  of  size  is  less  striking 
(Fig.  2,  <7,  I,  g,  £).  The  larger  one  is  about  equal  in  size- to  the 
smallest  of  the  peripheral  chromosomes  and  hence  cannot  be 
certainly  distinguished  when  it  lies  in  the  outer  ring  (Fig.  26). 
In  both  species  an  equatorial  plate  occasionally  occurs  in  which 
nine  chromosomes  clearly  appear  (Figs,  id,  2<r),  but  this  is  excep- 
tional, and  I  have  never  found  a  spindle  showing  this  body  in 
division.  The  presence  of  this  additional  chromosome  is  probably 
due  to  a  failure  of  synapsis  between  two  of  the  spermatogonial 
chromosomes  which  normally  conjugate  to  form  a  bivalent  body, 
and  it  is  evidently  to  be  regarded  as  an  abnormal  condition. 


lCf.  Montgomery's  Fig.  105,  of  Corizus,  '01,  i. 


Studies  on  Chromosomes. 


377 


•  ••  • 
*• 


m 


n  o 

Fio.  i.1 

Lygaeus  turcicus.  a,  e,  metaphase  of  first  division — in  the  first  two  figures  several  of  the  chromo- 
somes are  represented  out  of  their  natural  positions  at  one  side;  b,  c,  normal  melaphase-groups  in  polar 
view,  first  division;  d,  abnormal  form  with  nine  chromosomes;  /,  the  idiochromosomes  and  two  others, 
in  early  anaphase,  first  division;  g,  h,  daughter-groups,  late  anaphase  first  division,  from  the  same 
spindle;  /',  j,  equatorial  plates  of  second  division,  at  the  metaphase,  in  polar  view;  k,  prophase  of  second 
division,  showing  all  the  chromosomes  just  before  taking  up  their  definitive  positions;  /,  metaphase  of 
second  division — three  of  the  chromosomes  drawn  out  of  position  at  one  side;  m,  separation  of  the  idio- 
chromosomes, second  division;  n,  o,  daughter-groups,  late  anaphase  of  second  division,  from  the  same 
spindle. 


'All  of  the  figures  were  drawn  as  carefully  as  possible  with  the  camera,  a  TS  oil  immersion,  and 
compensation  ocular  12  (Zeiss),  enlarged  ^\  diameters  with  a  drawing  camera,  carefully  corrected 
by  renewed  comparison  with  the  objects  and  then  reduced  in  the  engraving  to  one-half.  At  such  an 
enlargement  some  error  is  unavoidable,  but  great  care  has  been  taken  to  represent  the  chromosomes 


37$  Edmund  B.  Wilson. 

In  side  views  of  the  spindles  both  species  usually  show  the  chro- 
mosomes of  a  symmetrical  dumb-bell  shape  (Figs,  la,  e,  2cT), 
though  one  or  more  of  them  may  appear  quadripartite,  as  is  espe- 
cially common  in  case  of  the  largest  one  or  macrochromosome 
(Fig.  2b,  i).  In  both  forms  all  of  the  eight  chromosomes  are 
symmetrically  divided  in  the  first  mitosis  (Figs,  if,  2e,  /),  giving 
rise  to  two  exactly  similar  daughter-groups  of  eight  chromosomes 
each  (Figs,  i^,  h,  2g,  /?).  The  rate  at  which  the  daughter- 
chromosomes  separate  varies  widely  in  different  cases.  Fre- 
quently the  idiochromosomes  lead  the  way  in  the  march  toward 
the  poles  and  may  be  widely  separated  at  a  time  when  one  or  more 
of  the  larger  chromosomes  are  only  just  separating  (Fig.  I/), 
while  the  macrochromosome  often  lags  behind  the  others  (Fig.  2*); 
but  now  and  then  a  spindle  shows  the  reverse  condition,  the  small 
idiochromosome  being  the  slowest  of  the  group.  In  the  end  the 
daughter-chromosomes  come  to  lie  at  the  same  level,  and  in  the 
final  anaphase  are  drawn  more  closely  together.  At  this  period 
the  grouping  of  the  chromosomes  is  exactly  the  same  in  the  two 
species,  the  two  idiochromosomes  lying  close  together  and  closely 
surrounded  by  a  ring  formed  by  the  six  larger  chromosomes.  In 
spindles  that  lie  vertically  or  slightly  obliquely  the  two  daughter- 
groups  may  in  both  species  be  seen,  with  the  greatest  clearness,  to 
be  exact  duplicates  of  each  other,  proving  beyond  doubt  the  equal 
division  of  all  of  the  eight  chromosomes  of  the  first  mitosis  (Figs. 
i£,  h,  2g,  /?),  and  the  ske-relations  of  the  chromosomes  persist 
without  noticeable  change.  In  Lygaeus  at  this  period  the  two 
idiochromosomes  are  still  as  a  rule  clearly  separated;  in  Ccenus 
this  may  be  the  case,  but  they  sometimes  lie  in  close  contact, 
already  forming  a  dyad  almost  identical  in  appearance  with  the 
central  unequal  dyad  of  the  second  mitosis  (Fig.  2/?). 

as  accurately  as  possible,  and  none  of  the  figures  are  schematized  in  this  respect,  except  that  in  a  few 
cases  one  or  more  of  the  chromosomes  have  been  drawn  out  of  their  natural  positions  in  order  to  avoid 
confusion  in  the  figures.  It  should  be  noted  that  in  the  division-stages  there  is  some  variation  in  the 
actual  size  of  the  chromosomes,  and  this  is  more  or  less  exaggerated  in  the  figures  owing  partly  to  slight 
differences  in  position,  which  cause  foreshortening  in  various  degrees,  and  partly  to  differences  in 
form  (different  degrees  of  elongation  of  the  chromosomes  cause  corresponding  variations  in  thickness 
as  seen  in  polar  view).  No  attempt  has  been  made  to  represent  the  minuter  details  of  the  spindle- 
fibers  or  asters,  though  the  figures  are  but  slightly  schematized  in  this  respect.  Some  of  the  stages  of 
the  growth-period  (especially  stages  b,  d  and  /)  are  difficult  to  represent  adequately  in  pen  drawings; 
though  I  have  attempted  to  show  them  as  accurately  as  possible. 

In  all  the  figures  the  idiochromosomes  are  marked  ;',  the  plasmosome  p. 


Studies  on  Chromosomes. 


379 


In  the  stage  that  immediately  follows,  the  chromosomes  for  a 
brief  period  become  so  crowded  that  their  exact  changes  cannot  be 
followed.  In  slightly  later  prophases  of  the  second  division,  which 
follows  without  a  pause,  the  chromosomes  again  spread  apart, 


a 


^^  ^^m 

*•• 


•  * 


m 


n 


o 


FIG.  2. 


Coenus  delius.  a,  b,  normal  equatorial  plates,  metaphase  of  first  division;  c  ,  abnormal  form  with  nine 
chromosomes;  d,  the  entire  chromosome-group,  first  division  metaphase,  in  side  view,  the  upper  four  in 
their  natural  position;  e,  /,  anaphases  in  side  view  showing  in  each  case  the  idiochromosomes  and  three 
others;  g,  h,  daughter-groups,  late  anaphase  of  first  division,  from  the  same  spindle;  i,  j,  equatorial 
netaphase  of  second  division,  polar  view;  k,  metaphase  of  second  division;  /,  later  metaphate, 
separation  of  the  idiochromosomes;  m,  n  —  o,  p,  two  pairs  of  daughter-groups,  late  anaphase,  second 
di\  1'ion,  in  each  case  from  the  same  spindle. 


380  Edmund  B.  Wilson. 

and  it  may  now  be  seen,  even  before  the  equatorial  plate  is  formed, 
that  the  two  idiochromosomes,  retaining  their  characteristic  size- 
relations,  have  conjugated  to  form  an  asymmetrical  dyad,  which 
shows  the  most  striking  contrast  to  the  six  other  dyads,  all  of 
which  have  a  symmetrical  dumb-bell  shape  (Fig.  i&).  The  seven 
dyads  are  now  drawn  into  the  equatorial  plate,  the  asymmetrical 
one  invariably  lying  at  the  center  of  a  ring  formed  by  the  six 
symmetrical  ones  (Figs.  I/,  m,  2k,  36).  These  dyads  place  them- 
selves with  their  long  axes  parallel  to  that  of  the  spindle,  so  that 
when  seen  in  polar  view  they  present  a  circular  or  more  or  less 
ovoidal  outline;  if  they  lie  in  a  slightly  oblique  position,  as  is  fre- 
quently the  case,  they  may  give  a  bipartite  appearance.  Since  in 
polar  view  the  small  idiochromosome  lies  above  or  below  the  large 
one  it  is  usually  invisible,  and  hence  only  the  larger  one  appears  at 
the  center  of  the  equatorial  plate  (Figs,  li,  j,  21,  /).  In  the  early 
metaphase  the  chromosomes,  especially  in  Coenus,  are  often 
rather  widely  separated,  so  that  the  equatorial  plate  may  be 
nearly  or  quite  as  wide  as  in  the  first  division  (cf.  Figs.  2a,  b,  21,  /). 
Such  figures  might  at  first  sight  readily  be  mistaken  for  those  of 
the  first  division,  but  without  exception,  in  my  material,  both  the 
chromosomes  and  the  cell-bodies  of  the  y-chromosome  cells  are 
much  smaller  than  those  of  the  8-chromosome  ones;  and  the  com- 
pleteness of  the  series  and  the  great  number  of  division-figures 
that  I  have  had  under  observation  precludes,  I  think,  the  possi- 
bility of  error  on  this  point. 

In  the  ensuing  division  each  of  the  dyads  draws  apart  into  two 
spheroidal  single  chromosomes,  the  peripheral  ones  dividing 
equally,  while  the  idiochromosome-dyad  separates  into  its  two 
unequal  constituents — invariably,  I  believe,  leading  the  way  in 
the  division  (Figs.  Ira,  2/,  %c,  g,  &).  Owing  to  this  fact  the  unequal 
division  of  the  central  dyad  may  be  seen  with  unmistakable  clear- 
ness. In  polar  or  slightly  oblique  view  of  the  late  anaphases, 
when  both  daughter-groups  are  visible,  the  asymmetrical  result 
may  plainly  be  seen.  In  such  figures  the  two  daughter-groups 
show  a  most  striking  contrast  to  those  of  the  first  division,  being 
no  longer  duplicates  of  each  other,  and  both  showing  seven  instead 
of  eight  chromosomes.  Each  daughter-group  shows,  as  in  the 
metaphase-group,  a  ring  of  six  larger  chromosomes  (now  single 
spheroidal  bodies)  within  which  lies  at  one  pole  the  smaller,  at 
the  other  pole  the  larger,  of  the  idiochromosomes  (Figs,  in,  o,  2m, 


Studies  on  Chromosomes. 


38 


n,  o,  /?).  Each  spermatid-nucleus  thus  receives  seven  chromo- 
somes, one-half  the  spermatogonial  number,  and  no  accessory 
chromosome,  in  the  usual  sense  of  the  word,  is  present;  but  the 
spermatids  nevertheless  consist  of  two  groups,  equal  in  number,  one 
of  which  contains  the  smaller,  the  other  the  larger  of  the  idiochro- 
mosomes.  In  the  mature  spermatozoa  I  have  not  been  able  to 
detect  any  corresponding  difference. 


a 


d 


f  • 


b 


•••       M 


I 


m 

FIG.  3. 

Euschistus  fissilis,  Euschistus  sp.,  Podisus  spinosus.  a-c,  Euschistus  fissilis.  a,  metaphase- 
group,  first  division;  b,  c,  metaphase-figure,  and  early  anaphase  in  side  view,  second  division,  d-i, 
Euschistus  sp.;  d,  e,  metaphase  groups,  first  division;  /,  metaphase-group,  second  division;  g,  second 
division,  side  view;  h,  i,  daughter-groups,  late  anaphase  of  second  division,  from  the  same  spindle; 
j-m,  Podisus  spinosus;  j,  metaphase-group,  first  division;  k,  late  anaphase,  second  division;  /,  m, 
daughter-groups  from  the  same  spindle,  late  anaphase,  second  division. 

In  Euschistus,  Brochymena,  Podisus  and  Trichopepla  the  facts 
are,  with  a  few  variations  of  detail,  essentially  similar.  In  both 
species  of  Euschistus  and  in  Brochymena  the  number  of  chromo- 


382 


Edmund  B.  Wilson, 


somes  agrees  with  that  of  Lygaeus,  being  14  in  the  spermatogonia, 
eight  in  the  first  spermatocyte-division,  and  seven  in  the  second, 
and  their  grouping  is  similar  (Figs.  3,  7),  save  that  in  Brochymena, 
alone  among  all  the  forms,  the  small  idiochromosome  frequently 
lies  in  the  outer  ring  (Fig.  7&).  Podisus  differs  only  in  the 
fact  that  the  numbers  are  respectively  9  and  8  (Figs.  3/,  /,  ra), 
while  the  spermatogonial  number  is  16  instead  of  14  (Fig.  5^). 
Trichopepla  is  a  puzzling  case  which  I  have  not  yet  fully  cleared 
up.  The  daughter-groups  of  the  second  division  show  7  chromo- 


C 


*•• 

»      t 


FIG.  4. 

Nezara.  a,  b,  metaphase-groups,  first  division;  c,  metaphase-group,  second  division;  d,  entire  chro- 
mosome-group, second  division,  in  side  view,  showing  equal  division  (three  chromosome-pairs  from  a 
lower  level  of  the  spindle  shown  at  the  right);  e,  f,  duplicate  sister  chromosome-groups,  anaphase  of 
second  division,  from  the  same  spindle;  g,  h,  spermatogonial  metaphase-groups. 

somes  each  (Fig.  Jq,  r),  the  idiochromosomes  being  well  differ- 
entiated. The  first  division,  however,  agrees  with  Montgomery's 
description  in  showing  either  9  (Fig.  70)  or  8,  the  smallest  chro- 
mosome being  in  the  latter  case  wanting. 

The  conditions  in  Nezara  are  of  particular  interest  from  a 
comparative  point  of  view  in  that  the  idiocbromosomes  are  of  equal 


Studies  on  Chromosomes.  383 

size.  The  first  spermatocyte-division  shows  8  chromosomes, 
which  differ  in  grouping  from  that  of  the  other  forms  in  that  all 
usually  lie  in  a  ring  without  a  chromosome  at  its  center  (Fig.  4 
a,  />).  For  this  reason  no  clue  to  the  identification  of  the  idiochro- 
mosomes  is  given  by  their  position.  Two  of  the  chromosomes  are, 
however,  distinctly  smaller  than  the  others;  and  the  relations  in 
the  spermatogonia  leave  little  doubt  that  it  is  these  two  that  corre- 
spond to  the  idiochromosomes.  They  may  lie  side  by  side  (Fig. 
4*3)  or  more  or  less  widely  separated  (4^).  All  these  chromosomes 
are,  in  side  view,  seen  to  have  a  symmetrical  dumb-bell  shape,  and 
all  are  equally  halved  in  the  first  division.  None  of  the  prepara- 
tions show  the  stage  immediately  following;  but  there  can  be  no 
doubt  that  a  conjugation  of  the  two  small  chromosomes  takes 
place  at  this  time,  since  the  second  division  (of  which  I  have  a  large 
number)  invariably  shows  in  polar  view  but  7  chromosomes,  which 
have  now  assumed  the  usual  arrangement,  with  one  in  the  center 
of  a  ring  formed  by  the  6  others  (Fig.  4<r).  Nezara  differs  again, 
however,  from  all  the  other  forms  in  the  fact  that  the  small  chro- 
mosome (/'.  e.,  the  idiochromosome-dyad)  lies  in  the  outer  ring, 
in  many  if  not  in  all  cases,  while  one  of  the  larger  chromosomes 
lies  at  the  center  of  the  ring.  Lateral  views  of  the  spindle  show 
all  of  the  chromosomes  as  quite  symmetrical  dyads;  and  in  the 
ensuing  division  all  divide  equally  (Fig.  4^).  In  such  views  the 
idiochromosome-dyad,  which  is  readily  recognizable  by  its  size, 
may  be  clearly  seen  to  divide  equally;  and  in  this  respect  Nezara 
differs  from  all  the  others.  The  anaphase  sister-groups  of  the 
same  spindle,  each  containing  7  chromosomes  (Fig.  4^,  /)  are, 
accordingly,  exact  duplicates,  the  idiochromosome  retaining  its 
position  in  the  outer  ring. 

The  symmetrical  division  of  the  idiochromosome-dyad  in  Nez- 
ara is  a  fact  of  importance  for  the  comparison  of  the  idiochromo- 
somes with  the  microchromosomes  of  such  forms  as  Anasa  or 
Alydus.  Were  it  not  for  their  failure  to  unite  to  form  a  bivalent 
body  until  the  end  of  the  first  mitosis  we  should  find  no  ground  in 
this  case  for  designating  these  chromosomes  by  a  special  name. 

2       The  Spcrmatogonial  Chromosomes. 

We  have  now  to  examine  the  relation  of  the  chromosomes  of 
the  first  maturation  to  those  of  the  spermatogonia.  The  material 


384  Edmund  E.  Wilson. 

for  the  spermatogonial  division  is  most  abundant  in  the  case  of 
Lygaeus,  which  is  much  the  most  favorable  form  for  an  accurate 
count,  the  chromosomes  being  well  separated  and  showing  with 
almost  schematic  clearness.  In  the  preparations  of  this  form 
numerous  spermatogonial  plates  appear,  showing,  whenever  an 
accurate  count  can  be  made,  without  exception  14  chromosomes 
(Fig.  5^,  /?).  Nezara,  of  which  numerous  spermatogonial 
divisions  are  also  available,  shows  the  same  number  and  with 
almost  equal  clearness,  though  the  chromosomes  are  in  this  form 
more  crowded.  In  the  other  forms  the  material  is  less  abundant, 
but  the  relations  are  clearly  shown  in  most  of  them.  Coenus 
(Fig.  5/)  Euschistus  (Fig.  5y),  and  Brochymena  (Fig.  777)  also 
show  14  spermatogonial  chromosomes,  while  in  Podisus  (Fig.  5^) 
the  number  is  16. *  In  all  these  cases,  therefore,  the  spermat- 
ogonial number  is  double  that  of  the  chromosomes  in  the  second 
spermatocyte-division,  and  two  less  than  double  the  number  in 
the  first  division.  The  most  striking  fact  is  that  in  all  these 
forms,  with  the  exception  of  Nezara,  the  spermatogonial  groups 
show  but  one  microchromosome  (marked  i  in  Figs.  4,  5,  7); 
in  striking  contrast  to  the  fact  first  determined  by  Paulmier  in 
Anasa  and  afterward  by  Montgomery  in  many  other  Hemiptera, 
that  two  such  bodies  ("chromatin-nucleoli"  of  Montgomery) 
equal  in  size,  are  often  present.  Did  this  observation  rest  only  on 
the  examination  of  a  few  division-figures  (as  in  the  case  of  Coenus, 
Euschistus,  Podisus  and  Brochymena)  I  should  hardly  trust  in  its 
general  applicability  to  the  species;  but  in  Lygaeus  numerous 
demonstrative  cases  remove  every  doubt  regarding  this  point,  and 
the  agreement  of  the  other  forms  in  their  later  history  makes  it 
nearly  certain  that  my  observation  is  not  at  fault  with  them.2 

Distinct,  though  not  very  great,  size-differences  may  be  observed 
in  the  larger  spermatogonial  chromosomes,  as  has  been  indicated 
by  Montgomery  in  several  other  genera  of  Hemiptera.  Though 
these  differences  are  not  nearly  as  marked  as  those  recognized 
by  Sutton  in  Brachystola,  it  is  nevertheless  pretty  clearly  evident 


JMontgomery  ('01,  i)  gives  the  numbers  as  follows:  Euschistus  variolarius  16,  E.  tristigmus  14 
Nezara  16,  Ccenus  14,  Brochymena  16,  Podisus  16. 

2This  point  is  emphasized  since  Montgomery  describes  and  figures  two  spermatogonial  microchro- 
mosomes  ("chromatin-nucleoli1')  in  Euschistus  variolarius  ('01,  i,  Figs.  2,  3),  E.  tristigmus  (pp.  cit., 
Fig.  20),  Coenus  delius  (Fig.  55),  and  Brochymena  (Fig.  47).  One  of  the  figures  of  the  first-named 
species  shows  them  equal,  the  other  unequal,  in  size. 


Studies  on  Chromosomes. 


385 


that  the  larger  chromosomes  may  be  grouped  in  six  pairs;  and  in 
Figs.  5/7,  /,  /,  4£,  Jnl  I  have  attempted  to  indicate  these  by  corre- 
sponding numbers;  though  no  pretense  to  complete  accuracy  of 
identification  can  be  made,  since  the  chromosomes  vary  somewhat 
in  form  and  their  apparent  sizes  vary  somewhat  with  their  posi- 
tion, owing  to  foreshortening.  Allowing  for  all  errors  of  identifica- 
tion it  is  obvious  that  in  all  but  Nezara  12  of  the  larger  chromo- 


g 


1\ 


h 


FIG.  5. 

Coenus  (a-j,  »'),  Lygaeus  (g,  h*),  Euschistus  (y),  Podisus  (A),  a,  Coenus,  contraction-phase,  showing 
both  idiochromosomes  in  the  form  of  chromosome-nucleoli;  b,  prophase,  corresponding  to  stage  h  in 
Lygaeus,  the  two  idiochromosomes  and  four  of  the  others;  c-f,  chromosome-nucleoli  of  Coenus  from  a 
stage  corresponding  to  stage  f;  c,  d,  both  idiochromosomes  present,  c,  f,  single  chromosome-nucleoli; 
g,  h,  spermatogonial  metaphase-groups,  Lygaeus;  /,  spermatogonial  group,  Coenus;  j,  spermatogonial 
group,  Euschistus,  sp.;  k,  spermatogonial  group,  Podisus. 

somes  may  be  symmetrically  paired  ofF,  two  by  two,  while  a 
thirteenth  (unnumbered  in  the  figures)  is  left  without  a  fellow  of 
like  size.  The  conclusion  is,  I  think,  irresistible  that  in  synapsis 


'In  the  last  figure  the  chromosomes  have  been  by  inadvertence  wrongly  lettered. 


386  Edmund  B.  Wilson. 

twelve  of  the  larger  chromosomes  unite  to  form  the  six  peripheral 
bivalents  of  the  first  division,  that  the  thirteenth  is  the  larger 
idiochromosome  and  the  fourteenth  (the  microchromosome)  the 
small  idiochromosome.  These  two  alone  remain  separate  in  the 
first  division  as  univalent  chromosomes,  thus  giving  a  group  of 
eight  instead  of  seven.  This  conclusion  is  in  accordance  with 
Montgomery's  interpretation  of  the  facts  observed  by  him  in 
Euchistus  tristigmus,  as  pointed  out  beyond,  except  that  he 
believed  that  in  this  form  the  first  division  might  in  many  cases 
show  only  seven  chromosomes,  the  "chromatin  nucleoli"  having, 
like  the  other  chromosomes,  already  conjugated  to  form  a  bivalent 
body.  My  interpretation  is  strikingly  confirmed  by  the  facts 
observed  in  Nezara;  for  in  this  case,  where  the  first  spermatocyte- 
division  shows  two  chromosomes  of  equal  size  and  the  smallest  of 
the  group,  the  spermatogoma  correspondingly  show  two  equal 
microchromosomes,  as  in  Anasa,  Alydus,  or  Protenor  (Fig.  4^,  /?). 
Since  these  two  are  represented  in  the  second  division  by  a  single 
symmetrical  dyad,  which  is  again  the  smallest  of  the  group 
(Fig.  4  g—f)  it  is  evident  that  the  two  equal  microchromosomes 
must  conjugate  after  the  first  division.  They  therefore  agree  in 
behavior  with  the  unequal  idiochromosomes  present  in  the  other 
forms,  and  differ  from  those  of  the  Anasa  type,  in  remaining 
separate  during  the  first  maturation-mitosis. 

?.      The  Growth-Period  of  the  Primary  Spermatocytes. 

(a)  General  History. 

It  is  not  my  purpose  to  describe  in  detail  at  this  time  the  general 
history  of  the  chromatin  during  the  growth-period,  but  it  will  be 
convenient  to  outline  the  stages  in  order  to  trace  the  history  of  the 
idiochromosomes.  In  Lygaeus  ten  well  marked  stages  may  be 
distinguished  from  the  early  synapsis  (a)  to  the  metaphase  of  the 
first  division  (/),  inclusive.  Though  some  of  these  stages  are  best 
characterized  by  the  condition  of  the  idiochromosomes,  an  account 
of  the  latter  can  more  readily  be  given  after  considering  the  history 
of  the  larger  chromosomes.  In  stage  a  (early  synapsis),  which 
shortly  follows  the  final  anaphase  of  the  last  spermatogonial 
division,  the  chromosomes  have  the  form  of  rather  ragged,  longi- 
tudinally split  loops,  the  free  ends  of  which  converge  toward 


387 


Studies  on  Chromosomes. 


one  pole  of  the  nucleus.  In  stage  b  (contraction-phase,  Fig. 
they  become  closely  massed  in  a  spheroidal  aggregation  at  the 
center  or  toward  one  side  of  the  nucleus,  and  surrounded  by  a 
large  clear  space.  At  this  time  they  stain  so  deeply  that  their 


FIG.  6. 

Lygaeus  turcicus.  a,  contraction-phase  of  synaptic  period  (stage  fe)  showing  large  idiochromosome; 
b,  early  post-synapsis  (stage  c\  both  idiochromosomes  shown;  c,  stage  d,  a  plasmosome  connected  with 
the  large  idiochromosome;  d-f,  chromosome-nucleoli  (with  plasmosome)  from  stage  e;  d  and  e  single, 
/,  showing  the  two  idiochromosomes;  g,  stage  /,  with  plasmosome  at  its  maximum  size,  and  both  idio- 
chromosomes; h-n,  chromosome-nucleoli  from  stage  /  (represented  side  by  side  near  the  plasmosome, 
out  of  their  natural  position);  h-k,  four  cases  in  which  but  one  is  present,  l-n  showing  both  idiochromo- 
somes; o,  stage  g,  with  both  idiochromosomes;  p,  q,  stage  h,  two  sections  of  the  same  nucleus  showing  all 
of  the  eight  chromosomes  and  a  small  plasmosome;  r,  stage  ;,  showing  the  six  larger  chromosomes  and 
the  two  idiochromosomes. 


388  Edmund  B.  Wilson. 

outlines  can  only  with  difficulty  be  made  out,  and  then  only  after 
long  extraction.  In  stage  c  (early  post-synapsis)  they  again 
spread  through  the  nuclear  cavity,  giving  the  appearance  of  an 
interrupted,  contorted  and  more  or  less  confused  spireme,  com- 
posed of  delicate  and  somewhat  varicose  threads  (Fig.  6/>).  In 
stage  d  (Fig.  6<r)  the  threads  become  somewhat  shorter  and  thicker 
and  have  a  more  open  arrangement,  but  still  show  a  very  marked 
spireme-like  arrangement.  I  believe  the  threads  to  be  at  this 
period  longitudinally  split,  though  this  can  only  be  made  out  with 
difficulty.  This  stage  is  well  characterized  by  the  condition  of  the 
large  idiochromosome,  as  described  beyond,  and  by  the  appear- 
ance of  a  pale  plasmosome.  In  stage  e  the  threads  become  mark- 
edly shorter  and  thicker,  ragged  in  outlines,  often  faintly  show  a 
longitudinal  split,  and  diminish  somewhat  in  staining-capacity. 
The  chromosomes  now  undergo  a  great  change,  becoming  in 
stage  /  very  loose  in  texture,  showing  only  vague  boundaries,  and 
almost  completely  losing  their  staining-capacity,  so  that  it  is 
difficult  to  represent  their  appearance  in  a  black  and  white  figure 
(Fig.  6^).  As  may  be  seen  from  the  figure  the  chromosomes 
now  give  the  appearance  of  a  rather  vague,  pale,  finely  granular 
network,  in  which  traces  of  a  spireme-like  arrangement  can 
usually  be  seen,  but  they  cannot  be  clearly  made  out  as  individual 
bodies.  Stage  g  again  shows  a  great  change  (Fig.  60)  the  chro- 
mosomes having  resumed  their  staining  capacity  and  definite 
boundaries,  and  now  appearing  as  long,  coarse,  winding  threads, 
often  showing  rather  ragged  outlines  and  a  more  or  less  distinct 
longitudinal  split,  as  in  stage  e;  the  two  stages  may,  however,  at 
once  be  distinguished  both  by  the  position  of  the  cells  in  the  testis 
and  by  the  different  relation  of  the  plasmosome  and  the  chromo- 
some-nucleoli,  as  described  below.  The  condensation  of  the 
chromosomes  now  takes  place,  the  succeeding  three  stages  follow- 
ing in  rapid  succession.  In  stage  h  the  long  split  rods  shorten 
and  thicken,  stain  much  more  deeply,  and  assume  a  great  variety 
of  forms — curved  double  rods,  dumb-bell  shaped  figures  (some- 
times longitudinally  split),  closed  rings,  and  peculiar  cross-forms. 
(Fig.  6p,  q,  which  show  all  the  chromosomes  from  a  single 
nucleus.)  In  stage  i  (early  prophase)  all  these  forms  condense 
into  quadripartite  tetrads  or  dyad-like  bodies,  the  latter  consisting 
of  two  symmetrical  halves  closely  joined  together  and  frequently 
showing  no  trace  of  a  second  division,  though  in  the  same  nucleus 


Studies  on  Chromosomes.  389 

may  occur  one  or  more  distinctly  quadripartite  forms  (Fig.  6r). 
The  history  of  these  dyad-like  bodies  clearly  shows,  I  think,  that 
with  the  exception  of  the  idiochromosomes  they  are  derived  from 
bivalent  longitudinally  split  rods,  and  they  have  therefore  the 
same  valence  as  the  actual  tetrads  with  which  they  are  associated. 
The  nuclear  membrane  now  disappears  and  the  chromosomes  are 
drawn  into  the  equatorial  plate  of  the  first  mitosis. 

The  general  history  of  the  growth-period  in  Coenus  is  similar  to 
that  of  Lygaeus,  but  is  in  some  respects  abbreviated;  and  stage  / 
is  much  less  marked,  the  chromosomes  not  losing  their  boundaries 
or  their  staining-capacity  in  so  great  a  degree,  and  still  presenting 
the  appearance  of  a  ragged  interrupted  spireme. 

(b)  The  Idiochromosomes  during  the  Growth-period. 

The  foregoing  description  applies  only  to  the  larger  or  ordinary 
chromosomes.  Throughout  the  whole  of  the  growth-period  in 
Coenus  and  from  stage  e  onward  in  Lygaeus,  at  least  one  of  the 
idiochromosomes  can  always  be  distinguished  as  a  compact, 
spheroidal,  intensely  staining  chromosome-nucleolus,  and  fre- 
quently both  idiochromosomes  are  distinguishable  in  this  form  in 
all  of  these  stages.  The  early  stages  of  Lygaeus  (b  to  */)  are  of 
especial  interest  in  that  the  condensation  of  the  idiochromosomes 
is  delayed,  and  at  least  the  larger  one  still  has  the  form  of  an  elon- 
gate, longitudinally  split  rod  or  thread.  Even  at  this  time,  however, 
it  is  immediately  distinguishable  from  the  others  by  its  greater 
thickness  and  greater  staining  capacity.  It  is  clear  beyond  all 
question,  therefore,  that  at  least  the  large  idiochromosome  may 
retain  its  identity  throughout  the  whole  growth-period.  With  the 
small  idiochromosome  the  case  is  not  so  strong,  as  will  be  seen 
from  the  following  account. 

It  is  convenient  to  trace  the  history  of  the  idiochromosomes  in 
the  reverse  order  from  stage  i  backward,  again  taking  Lygaeus 
as  the  type.  In  this  stage  (late  prophase),  when  the  six  larger 
chromosomes  are  in  the  form  of  condensed  tetrads  or  dyad-like 
bodies,  both  the  idiochromosomes  have  very  distinctly  the  form 
of  dyads.  The  nucleus  now  contains  therefore  eight  separate 
chromosomes,  among  which  the  idiochromosomes  are  at  once 
recognizable  by  their  small  size  (6r).  In  stage  h  the  idiochromo- 
somes have  the  same  general  appearance,  though  their  bipartite 


39°  Edmund  B.  Wilson. 

form  is  often  less  distinct  (6*7).  Up  to  this  point  both  idiochro- 
mosomes  are  apparently  always  present.  In  the  stages  now  to  be 
considered  both  are  often  plainly  distinguishable,  but  quite  as 
frequently  this  is  not  the  case,  the  nucleus  showing  but  a  single 
chromosome-nucleolus  the  size  of  which  proves  it  to  be  either  the 
large  idiochromosome  or  the  large  and  small  one  united.  Between 
these  two  possibilities  I  have  not  been  able  to  decide  in  Lygaeus 
and  Coenus;  but  decisive  evidence  is  given  in  the  case  of  Brochy- 
mena,  as  described  beyond.  In  stagey  (60)  the  idiochromosomes 
(or  the  single  chromosome-nucleolus)  appear  as  spheroidal  com- 
pact bodies,  usually  not  showing  a  bipartite  structure,  and  in 
addition  one  or  more  pale  rounded  plasmosomes  are  often  present. 
In  stage  /,  owing  to  the  loss  of  staining  capacity  by  the  larger 
chromosomes,  the  idiochromosomes  show  with  brilliant  clearness, 
since  they  are  still  stained  intensely  black,  and  may  very  readily 
be  studied  with  care  (6g-n~).  When  both  are  present  they  appear 
slightly  larger  than  in  the  later  stages,  and  often  the  larger  one 
is  plainly  seen  to  be  hollow  (which  probably  accounts  for  its  larger 
size)  though  this  is  shown  still  more  clearly  in  Brochymena,  as 
described  beyond.  A  small  plasmosome  is  sometimes  attached 
to  it  at  one  side,  but  in  addition  to  this  there  is  always  present  a 
very  large  pale  plasmosome  quite  free  from  both  idiochromosomes, 
and  free  also  from  the  single  chromosome-nucleolus  when  but 
one  of  these  bodies  is  present.  In  stage  e  the  idiochromosomes 
present  the  same  appearance,  but  the  larger  one  is  now  always 
attached  io  the  large  plasmosome,  and  often  more  or  less  flattened 
against  it  (as  is  also  the  single  chromosome-nucleolus  when  but 
one  appears)  while  the  small  one  is  almost  always  free  from  it 
(Fig.  6d,  e,  /).  The  larger  body  is  as  before  often  evidently 
hollow. 

Up  to  this  point  Lygaeus  and  Coenus  agree  almost  exactly.  In 
the  earlier  stages  they  differ  in  that  Coenus  still  shows  the  idio- 
chromosomes in  the  form  of  compact  chromosome-nucleoli, 
while  in  Lygaeus  the  larger  one  certainly,  and  I  believe  the  smaller 
one  also,  assumes  the  form  of  a  longitudinally  split  elongate  chro- 
mosome. In  stage  d  in  Lygaeus  (Fig.  6<:)  the  large  idiochromo- 
some is  a  rather  short,  deeply  staining  rod,  longitudinally  split, 
and  still  attached  (usually  toward  one  end)  to  the  plasmosome, 
which  is  now  considerably  smaller.  The  small  idiochromosome 
is  now  also  more  or  less  elongate,  but  I  cannot  be  sure  whether  it 


Studies  on  Chromosomes.  391 

is  longitudinally  split.  All  intermediate  stages  may  readily  be 
observed  between  this  stage  and  the  next  earlier  one  (<:)  in  which 
the  large  idiochromosome  is  an  elongate  split  thread  that  may 
extend  through  more  than  half  the  diameter  of  the  nucleus  (Fig. 
6£).  It  is,  however,  at  once  distinguishable  from  the  other  chro- 
mosomes by  its  straighter  course,  greater  thickness  and  deeper 
staining  capacity,1  which  renders  it  very  conspicuous  among  the 
thread-like  chromosomes.  No  plasmosome  can  now  be  seen. 
The  small  idiochromosome  can  still  clearly  be  distinguished  in 
many  of  the  nuclei  as  a  short  rod  staining  like  the  larger  one. 
Finally,  in  the  contraction-phase  (stage  />)  when  all  the  chromo- 
somes are  massed  together,  the  large  idiochromosome  still  unmis- 
takably appears  as  a  very  distinct  deeply  staining  rod,  sometimes 
nearly  straight,  more  usually  curved,  and  frequently  horse-shoe 
shaped,  at  one  side  of  the  chromosome-mass  (6#).  Neither 
plasmosome  nor  small  idiochromosome  can  now  be  made  out. 
In  Coenus  at  this  period  (Fig.  50)  both  idiochromosomes  (or  the 
single  chromosome-nucleolus)  still  appear  as  compact,  deeply 
staining  spheroidal  bodies,  the  larger  one  typically  having  a  small 
plasmosome  attached  to  it  at  one  side. 

The  early  history  of  the  large  idiochromosome  proves  most 
clearly  that  the  chromosome-nucleolus  into  which  it  afterward 
condenses  is  a  modified  chromosome  (as  Montgomery  first  showed 
in  Euschistus  variolarius)  and  one  that  forms  a  connecting  link 
between  the  ordinary  chromosomes  and  the  more  usual  forms  of 
"chromatin-nucleoli"  in  Hemiptera,  described  by  Montgomery, 
or  the  accessory  chromosome  of  Orthoptera;  for  in  none  of  these 
latter  is  the  condensation  to  a  compact  body  so  long  delayed. 
Paulmier  ('99)  showed,  however,  that  in  Anasa  the  compact  chro- 
mosome-nucleolus of  the  synaptic  period  afterward  elongates 
considerably  and  appears  as  a  rather  short  longitudinally  split 
rod,  similar  to  that  of  Lygaeus  at  stage  d  (Paulmier,  Fig.  22) 
afterward  again  condensing  into  a  compact  tetrad.  In  Lygaeus 
there  can  be  little  doubt  that  the  central  cavity  of  the  spheroidal . 
chromosome-nucleolus,  often  visible  in  stages  e-g,  represents  the 
original  longitudinal  split.  It  is  therefore  hardly  open  to  doubt 
that  the  division  of  the  large  idiochromosome  in  the  first  mitosis  is 
an  equation-division,  and  the  same  is  probably  true  of  the  small 

'This  has  been  somewhat  exaggerated  in  the  engraving. 


392  Edmund  B.  Wilson. 

idiochromosome.  It  is,  on  the  other  hand,  quite  certain  that  the 
division  of  the  idiochromosome-dyad  in  the  second  mitosis  is  a 
reduction-division.  The  order  of  the  divisions  in  case  of  the 
idiochromosomes  is  thus  the  reverse  of  that  which  occurs  in  the 
other  chromosomes,  according  to  Paulmier's  and  Montgomery's 
accounts;  and  as  pointed  out  beyond,  it  is  also  the  reverse  of  that 
which  takes  place  in  the  division  of  the  small  central  chromosome 
in  Anasa  and  Alydus. 

As  already  stated,  I  did  not  in  Lygaeus  and  Coenus,  succeed  in 
rinding  any  certain  explanation  of  the  fact  that  the  nuclei  of  the 
growth-period  may  show  either  one  or  two  chromosome-nucleoli. 

In  Brochymena,  however,  there  is  very  clear  evidence  on  this 
point.  Here,  too,  the  nuclei  of  the  middle  growth  period  show 
either  one  or  two  spheroidal  chromosome-nucleoli,  the  former  con- 
dition being  much  the  more  frequent.  When  both  are  present 
they  may  be  widely  separated  or  close  together,  and  both  very 
clearly  show  a  central  cavity,  which  is  rendered  very  conspicuous 
by  the  fact  that  the  chromatin  is  frequently  concentrated  in  a 
dark  zone  immediately  around  it  (Fig.  *]c-g).  When  but  one 
is  present  it  is,  as  a  rule,  perfectly  spherical,  hollow,  and  shows  no 
evidence  of  a  double  nature  (Fig.  7/,  g].  In  the  early  growth- 
period,  however,  the  single  chromosome-nucleolus  almost  always 
appears  bipartite,  being  composed  of  two  unequal  halves,  forming 
an  asymmetrical  dyad  (Fig.  Ja,  &)  very  similar  to  that  seen  in  the 
second  maturation-division  (Fig.  yra).  At  a  later  period  both 
of  the  constituents  become  hollow  (and  hence  appear  somewhat 
larger)  and  stain  less  deeply;  and  all  gradations  may  be  observed 
in  the  fusion  of  the  two  bodies  to  form  a  single  hollow'  body  (Fig. 
Jc,  d,  /)  which  is  plainly  as  large  as  the  two  separate  chromosome- 
nucleoli  (such  as  may  be  seen  in  cells  of  the  same  cyst)  taken 
together.  In  Brochymena,  therefore,  there  can  be  no  doubt  that 
when  only  one  chromosome-nucleolus  is  present  it  is  to  be  considered 
as  a  bivalent  body  arising  by  the  fusion  or  synapsis  of  the  two 
idiochromosomes. 

Thus  far  the  facts  confirm  the  interpretation  given  by  Mont- 
gomery ('01,  i)  who  observed  in  Coenus,  Euschistus  tristigmus 
and  some  other  forms  that  the  cells  of  the  growth-period  may  show 
either  a  single  "chromatin-nucleolus"  or  (in  Euschistus  "appar- 
ently more  frequently")  two  such  bodies  that  are  unequal  in  size; 
and  this  fact  he  interpreted  to  mean  that  the  two  corresponding 


Studies  on  Chromosomes. 


393 


spermatogonial  "chromatin-nucleoli"  may  either  conjugate  at  the 
period  of  general  synapsis  to  form  a  bivalent  body  or  may  remain 
separate  as  univalent  bodies.  As  already  pointed  out,  it  was 
probably  this  interpretation  that  led  him  to  conclude  that  the 
first  division  in  these  forms  might  show  either  seven  or  eight 
chromosomes.  But  the  later  stages  observed  in  Brochymena 
give  conclusive  evidence  that  even  though  such  a  primary  synapsis 


• *  :\  i » i 

09  m 


•  • 


o 


FIG.  7. 

Brochymena,  Trichopepla.  a-n,  Brochymena,  o-r,  Trichopepla;  a,  b,  idiochromosomes  and  plas- 
mosomes,  from  early  growth-period;  c-g,  condition  of  the  idiochromosomes  in  middle  and  late  growth- 
periods;  h,  prophase  of  first  division,  showing  idiochromosome-tetradj  i,  j,  two  stages,  one  following 
and  one  preceding  the  last,  in  the  division  of  the  bivalent  chromosome-nucleolus;  k,  metaphase-group, 
first  division;  /,  metaphase-group,  second  division;  tn,  side  view,  second  division,  showing  idiochromo- 
some-dyad  and  two  other  chromosomes;  n,  spermatogonial  metaphase-group;  o,  metaphase-group,  first 
division,  Trichopepla;  p,  spindle  of  second  division  in  lateral  view;  q,  r,  sister-groups,  late  anaphase  of 
second  division. 


394  Edmund  B.  Wilson. 

of  the  idiochromosomes  takes  place  the  bivalent  chromosome- 
nucleolus  again  separates  into  its  univalent  constituents  in  the  early 
prophases  of  the  first  maturation-division.  This  process,  which 
at  first  greatly  puzzled  me,  occurs  at  the  time  just  preceding  the 
concentration  of  the  larger  chromosomes  into  their  final  condensed 
form  (corresponding  to  stage  h  in  Lygaeus).  In  cysts  of  this  period 
every  stage  may  be  seen  in  the  transformation  of  the  single  chro- 
mosome-nucleolus  into  an  asymmetrical  tetrad,  consisting  of  two 
symmetrical  dumb-bell  shaped  bodies  of  unequal  size  (Fig.  77),  and 
the  separation  of  these  unequal  dyads  to  form  the  idiochromo- 
somes of  the  first  division  (cf.  Figs,  jh,  /,  y).  It  is  evident  that 
in  these  cases  the  final  reunion  or  conjugation  at  the  end  of  the 
first  division  is  not  a  primary  synapsis,  but  a  secondary  process.1 
The  facts  observed  in  Brochymena  make  it  very  probable  that 
when  only  a  single  chromosome-nucleolus  is  present  in  Lygaeus, 
Coenus  and  the  other  forms,  it  is  there  also  a  bivalent  body  as 
Montgomery  assumed;  but  the  uniform  separation  of  the  idio- 
chromosomes in  the  first  division  of  all  the  eight  species  I  have 
examined  is  almost  a  demonstration  that  in  all  the  forms  a  division 
of  the  bivalent  body  must  occur.  On  the  other  hand  it  seems 
equally  certain  that  in  many  of  these  forms  the  idiochromosomes 
may  fail  to  unite  at  the  period  of  general  synapsis,  and  may  remain 
separate  through  the  whole  growth-period;  and  in  Brochymena 
the  same  cysts  that  show  the  division  of  the  single  idiochromo- 
some-tetrad  may  also  contain  nuclei  in  which  the  dumb-bell 
shaped  idiochromosomes  are  widely  separated.  In  such  cases  it 
seems  probable  that  the  conjugation  of  the  idiochromosomes  at 
the  close  of  the  first  spermatocyte-division  must  be  regarded  as  a 
true  or  primary  synapsis  that  has  been  deferred  to  this  late  period. 

DISCUSSION    OF    RESULTS. 

The  most  essential  fact  brought  out  by  a  study  of  the  idiochro- 
mosomes is  that  in  Lygaeus,  Ccenus,  Podisus,  Euschistus,  Brochy- 
mena and  Trichopepla  a  dimorphism  of  the  spermatozoa  exists, 
there  being  two  groups  equal  in  number,  both  of  which  contain 

'The  division  of  the  bivalent  chromosome-nucleolus  is  similar  to  the  process  described  by  Gross 
('04)  in  Syromastes;  though  it  occurs  at  a  much  later  period.  For  reasons  that  will  appear  in  a 
subsequent  paper,  I  am,  however,  skeptical  in  regard  to  Gross's  conclusion  which  is  based  on  a  study 
not  of  the  idiochromosomes  but  of  paired  microchromosomes  similar  to  those  of  Anasa  or  Alydus. 


.         Studies  on  Chromosomes.  395 

the  same  number  of  chromosomes,  but  differ  in  respect  to  one  of 
them.  In  this  respect  these  genera  differ  from  all  those  that 
possess  an  accessory  chromosome  (Pyrrochoris,  Anasa,  Alydus, 
etc.),  since  in  the  latter  case  one-half  of  the  spermatozoa  receive 
one  chromosome  fewer  than  the  other  half.  It  is  remarkable 
that  two  types  of  dimorphism  apparently  so  different  should 
coexist  within  the  limits  of  a  single  order  of  insects.  We  are  thus 
led  to  inquire  into  the  relation  between  the  idiochromosomes  and 
the  accessory;  and  this  inquiry  must  also  include  the  "small 
chromosomes"  of  Paulmier  and  the  "chromatin-nucleoli"  of 
Montgomery.  It  will  conduce  to  clearness  if  the  second  part  of 
this  question  be  considered  first. 

It  is  evident  from  the  figures  and  descriptions  of  Montgomery 
('98,  '01,  i,  '01,  2,  '04)  that  the  bodies  I  have  called  idiochromo- 
somes are  identical  with  some  of  those  that  this  author  has  de- 
scribed under  the  name  of  "chromatin-nucleoli"  (which  are 
usually  also  small  chromosomes  or  microchromosomes);  but  it  is 
now  manifest  that  the  bodies  described  under  the  latter  name  are 
not  all  of  the  same  nature.  It  is  evident  that  two  types  of  these 
bodies  may  be  distinguished  that  differ  markedly  in  their  behavior 
during  the  maturation-mitosis.  One  of  these,  typically  repre- 
sented in  Anasa,  Alydus  and  Protenor  appear  in  the  spermat- 
ogonia  in  the  form  of  two  equal  microchromosomes  ("chromatin- 
nucleoli")  which  like  the  other  chromosomes  sooner  or  later  unite 
in  synapsis  to  form  a  bivalent  body  that  lies  at  the  center  of  the 
equatorial  plate  of  the  first  mitosis.  Both  divisions  accordingly 
show  exactly  one  half  the  spermatogonial  number  of  chromosomes 
but  it  is  a  very  noteworthy  fact  that  the  final  conjugation  of  the 
two  microchromosomes  is  long  deferred,  taking  place  in  the  propha- 
ses  of  the  first  division  (as  was  first  observed  by  Montgomery  in  Pro- 
tenor  and  some  other  forms,  more  recently  by  Gross  in  Syromastes 
and  by  myself  in  Anasa  and  Alydus).  In  this  case  there  seems  to 
be  no  doubt  that  the  first  division  of  the  bivalent  body  thus  formed 
is  a  reducing  division.  It  appears  to  be  further  characteristic  of 
this  type — at  least  in  all  the  forms  mentioned — that  a  true  acces- 
sory chromosome  is  associated  with  the  microchromosomes,  and 
that  only  one-half  of  the  spermatozoa  receive  one-half  the  somatic 
number  of  chromosomes,  the  other  spermatozoa  receiving  one 
less  than  this.  The  distinction  between  the  accessory  and  the 
microchromosomes  or  "chromatin-nucleoli"  first  demonstrated 


396  Edmund  B.  Wilson. 

by  Montgomery  ('01)  in  Protenor,  has  been  more  recently  shown 
by  Gross  ('04)  to  exist  in  Syromastes;  and  I  have  also  been  able  to 
demonstrate  the  same  fact  in  Alydus  and  in  Anasa  (Paulmier 
having  been  in  error  in  his  identification  of  the  accessory  with  the 
small  chromosome  in  the  last-named  form). 

The  second  type  includes  the  idiochromosomes,  which  in  the 
forms  I  have  studied  differ  from  the  Anasa  type  in  four  respects 
(Nezara  being  an  exception  in  regard  to  the  first  of  these),  namely, 
(l)  in  their  unequal  size,  with  which  is  correlated  the  fact  that  only 
a  single  microchromosome  appears  in  the  spermatogonia;  (2)  in 
the  fact  that  the  final  conjugation  or  synapsis  of  these  bodies  is 
deferred  until  the  prophases  of  the  second  division,  a  result  of 
which  is  that  the  first  division  shows  one  more  than  half  the  sper- 
matogonial  number  of  chromosomes  while  the  second  division 
shows  exactly  half  the  spermatogonial  number;  (3)  in  the  fact  that 
in  case  of  the  idiochromosomes  it  is  manifestly  the  second  division 
that  is  the  reducing  one,  while  my  observations  on  Lygaeus  render 
it  practically  certain  that  the  first  is  an  equation-division;  (4)  in 
the  fact  that  no  accessory  chromosome  in  the  usual  sense  is  present 
and  all  the  spermatozoa  receive  the  same  number  of  chromosomes. 
In  view  of  these  differences  it  seems  expedient  for  the  present  to 
place  these  two  types  in  different  categories. 

It  is  evident  from  Montgomery's  figures  and  descriptions  that 
he  observed  many  of  the  details  of  the  phenomena  described  in  the 
present  paper;  but  it  is  equally  clear  from  the  varying  interpreta- 
tions that  he  adopted  that  he  failed  to  reach  any  consistent  general 
result  regarding  the  behavior  of  the  idiochromosomes,  or  to  recog- 
nize the  dimorphism  of  the  spermatozoa.  For  example,  it  is 
evident,  I  think,  from  his  descriptions  of  Euschistus  variolarius, 
E.  tristigmus,  Coenus  delius,  Oncopeltus  fasciatus,  and  Lygus 
pratensis,  that  the  essential  facts  in  these  forms  agree  with  those 
I  have  described,  the  idiochromosomes  being  in  the  last  named 
two  species  of  equal  size,  as  in  Nezara;  but  Montgomery  offers 
for  each  of  these  cases  a  different  interpretation.  In  the  first 
named  species  the  idiochromosomes  are  clearly  figured  in  his  first 
paper  ('98,  Figs.  171,  188,  189,  etc.),  and  in  Fig.  214  they  are 
shown  separating  in  quite  typical  fashion  in  the  second  division 
(the  smaller  one  designated  as  a  "chromatin-nucleolus");  but  it 
is  evident  from  the  descriptions  given  in  both  this  and  the  following 
paper  ('01,  i)  that  he  did  not  reach  a  correct  interpretation  of  the 


Studies  on  Chromosomes.  397 

facts.1  In  Euschistus  tristigmus  and  Coenus  delius  the  first  divi- 
sion is  stated  to  show  either  seven  or  eight  chromosomes  (the 
spermatogonial  number  being  14),  but  quite  different  interpreta- 
tions are  given  of  this  in  the  two  species,  the  conditions  in  Euschis- 
tus being  assumed  to  be  due  to  the  frequent  failure  of  the  two 
"chromatin  nucleoli"  to  unite  in  synapsis,  while  in  the  case  of 
Coenus  seven  of  the  chromosomes  (including  the  large  "chromatin- 
nucleolus")  are  assumed  to  be  bivalent,  while  the  eighth  is 
an  additional  small  "chromatin-nucleolus"  not  distinguishable 

O 

in  the  spermatogonia  ('01,  I,  p.  166).  In  Euschistus  tristigmus 
the  "chromatin-nucleoli"  are  stated  to  be  of  unequal  size  and  to 
be  separated  from  each  other  without  divison  in  the  second 
mitosis.  This  is  evidently  the  same  phenomenon  that  I  have 
described;  though  Montgomery  overlooked  the  conjugation  of  the 
two  unequal  "chromatin-nucleoli"  at  the  end  of  the  first  division, 
and  expressly  states  that  they  are  not  joined  together  in  the  second 
division.  In  Oncopeltus,  likewise,  the  first  division  shows  one 
more  than  half  the  spermatogonial  number,  16  (i.  e.,  nine  instead 
of  eight,  precisely  as  I  have  described  in  Podisus),  and  this  is 
stated  to  result  from  the  persistence  of  the  two  "chromatin-nucle- 
oli" throughout  the  whole  growth  period  without  union;  but  an 
interpretation  differing  from  both  the  foregoing  is  here  sought  in 
the  assumption  that  each  of  the  two  "chromatin-nucleoli"  is 
bivalent,  even  in  the  spermatogonia  ('01,  I,  p.  186).  In  Lygus 
pratensis,  finally,  the  first  division  shows  18  chromosomes  and  the 
second  17,  the  still  different  explanation  being  here  offered  that 
the  two  "chromatin-nucleoli "  pass  undivided  one  to  each  pole  of 
the  first  spindle  ('01,  i).  Of  these  various  interpretations  only 
the  one  given  in  the  case  of  Euschistus  tristigmus,  I  believe,  con- 

'The  first  mitosis  is  here  clearly  shown  to  have  eight  chromosomes,  grouped  in  the  same  way  as  in 
my  "Euschistus  sp."  and  the  anaphase daughter-plate  of  the  second  division  is  shown  with  seven  (Fig. 
220),  precisely  as  in  the  two  species  I  have  studied.  Montgomery  gave  the  spermatogonial  number, 
correctly  I  believe,  as  14.  He  nevertheless  concluded  that  all  of  the  eight  chromosomes  (seven  chromo- 
somes +  i  "chromatin  nucleolus'')  divide  separately  in  both  divisions,  apparently  overlooking  the  fact 
that  this  would  give  the  spermatozoa  one  chromosome  too  many  (since  he  himself  demonstrated  that  the 
"chromatin-nucleolus"'  is  a  modified  chromosome).  This  account  of  the  divisions  is  not  modified  in 
the  paper  of  1901  except  in  the  statement  that  "in  the  second  maturation-division  the  chromatin-nucleo- 
lus is  not  always  divided"  (p.  161),  while  the  spermatogonial  number  is  now  given  as  16.  Since  the 
figures  of  the  earlier  paper  show  that  the  divisions  in  E.  variolarius  are  evidently  the  same  as  in  the 
species  I  have  examined,  I  think  that  on  both  these  points  the  first  account  was  probably  more  accurate 
than  the  later  one. 


398  Edmund  B.  Wilson. 

forms  to  the  true  one,  and  it  is  probable  that  all  of  these  cases 
will  be  found  to  agree  in  the  essential  phenomena  with  those  I 
have  determined  in  Lygaeus,  Coenus,  Nezara  and  the  other  forms. 

We  may  now  inquire  what  is  the  relation  of  the  idiochromo- 
somes  to  the  accessory  chromosome.  The  observations  suggest 
so  obvious  an  answer  to  this  question  that  I  wish  to  indicate  not 
only  the  evidence  in  its  favor,  but  more  especially  the  difficulties 
it  has  to  encounter.  In  forms  possessing  an  accessory  chromo- 
some the  spermatozoa  fall  into  two  equal  groups  that  differ  only 
in  respect  to  one  chromosome.  The  same  is  true  of  Lygaeus  and 
other  forms  that  lack  the  accessory  but  possess  the  idiochromo- 
somes,  with  the  difference  that  in  the  former  case  the  distinctive 
chromosome  is  present  in  but  one-half  the  spermatozoa,  while  in 
the  latter  case  two  such  distinctive  chromosomes  are  present,  one 
of  which  is  present  in  one-half,  the  other  in  the  other  half,  of  the 
spermatozoa.  It  is  impossible  to  overlook  the  evident  analogy 
between  the  two  cases;  and  the  idiochromosomes  may  in  one  sense 
be  considered  as  two  accessory  chromosomes  that  are  never  allotted 
to  the  same  spermatozoon  since  each  fails  to  divide  in  the  second 
mitosis  (precisely  as  is  the  case  with  the  single  accessory  in  other 
Hemiptera).  The  difference  between  Lygaeus  and  Coenus  in  the 
size-ratio  of  the  idiochromosomes  obviously  suggests  the  view 
that  the  single  accessory  of  other  forms  may  have  arisen  by  the 
disappearance  of  one  of  the  idiochromosomes;  and  in  Lygaeus  the 
smaller  one  is  already  so  minute  as  distinctly  to  suggest  a  vestigial 
structure.  We  might  accordingly  assume  that  in  a  more  primitive 
type  the  two  idiochromosomes  were  of  equal  size  (as  in  Nezara), 
undergoing  synapsis  and  subsequent  reduction  in  the  same  way 
as  the  other  chromosomes;  that  Coenus  and  Lygaeus  represent 
successive  stages  in  the  reduction  of  one  of  these  chromosomes; 
and  that  by  the  final  disappearance  of  the  smaller  one  in  such 
forms  as  Anasa  or  Pyrrochoris  a  single  accessory  chromosome 
remains. 

This  hypothesis  at  first  sight  seems  to  give  a  clear  and  intelli- 
gible view  of  the  origin  of  the  accessory  chromosome,  and  to  recon- 
cile the  remarkable  mode  of  spermatogenesis  occurring  in  the 
insects  with  forms  in  which  no  accessory  seems  to  appear.  But 
further  reflection  shows  that  it  has  to  encounter  a  formidable  if 
not  insuperable  difficulty  in  the  fact  that  in  some  of  the  forms 
possessing  an  accessory  chromosome  the  number  of  spermato- 


Studies  on  Chromosomes.  399 

gonial  chromosomes  is  an  even  one  (as  in  Anasa  and  Syromastes); 
and  there  seems  to  be  no  escape  from  the  conclusion  that  the  acces- 
sory is  here  a  bivalent  body  arising  by  the  synapsis  of  two  equal 
spermatogomal  chromosomes.  Even  in  cases  showing  an  odd 
number  of  spermatogonial  chromosomes  (as  in  many  Orthoptera 
and  some  Hemiptera — for  example  Alydus  or  Protenor)  it  has 
been  assumed,  and  with  good  reason,  that  one  of  the  chromosomes 
(probably  the  accessory)  is  already  bivalent,1  and  Montgomery 
has  shown  ('01,  i)  that  in  Protenor  the  large  accessory  ("chromo- 
some x ")  is  sometimes  transversely  constricted  into  two  equal 
halves  in  the  spermatogonia.  A  similar  fact  was  subsequently 
shown  in  Harmostes  ('01,  2)  which  also  has  normally  an  odd  sper- 
matogonial number.  To  this  should  be  added  the  fact  that  these 
forms  possess  the  small  bivalent  central  chromosome  (which 
arises  by  the  synapsis  of  two  equal  microchromosomes)  in  addition 
to  the  accessory.  The  difficulty  pointed  out  above  cannot  be 
escaped  by  supposing  that  the  disappearance  of  one  of  the  idio- 
chromosomes  has  been  effected  by  its  gradual  absorption  by  the 
other;  for  this  assumption,  too,  fails  to  explain  the  even  number 
of  spermatogonial  chromosomes.  Apparently  therefore  the  hypo- 
thesis I  have  suggested  must  in  the  present  state  of  our  knowl- 
edge be  considered  untenable.2 

It  appears  more  probable  that  the  idiochromosomes  are  com- 
parable   to    the    two   equal    microchromosomes    or    "chromatin- 

}Cf.  Montgomery,  '04. 

2Since  this  paper  was  sent  to  press  I  have  determined  beyond  the  possibility  of  doubt,  I  think,  that 
the  number  of  spermatogonial  chromosomes  in  Anasa  tristis  is  21,  not  22  as  given  by  both  Paul- 
mier  and  Montgomery.  This  result  is  based  on  the  study  of  a  large  number  of  preparations,  and 
careful  camera  drawings  of  more  than  twenty  perfectly  clear  metaphase  figures  have  been  made.  All 
without  exception  show  21  chromosomes,  and  I  have  sought  in  vain  for  even  a  single  cell  that  shows 
22.  (Paulmier's  original  slides  were  used.)  If  corroboratory  evidence  be  needed  it  is  given  by  the 
fact  that  there  are  always  three  macrochromosomes,  one  of  which  is  obviously  without  a  mate  of  like 
size,  and  is  probably  the  accessory.  I  have,  also,  positively  determined  the  spermatogonial  number 
to  be  21  in  a  form  included  in  Paulmier's  material  and  labeled  "  Chariesterus  antennator,' '  (since 
this  number  disagrees  with  Montgomery's  co'unt  of  the  spermatocytes  there  may  be  an  error  of  iden- 
tification; but  the  form  is  certainly  different  from  Anasa)  and  15  in  Archimerus  calcarator  (from  my 
own  material,  identified  by  Mr.  Uhler),  both  members  of  the  same  family  as  Anasa.  This  wholly 
unexpected  result  perhaps  justifies  a  certain  skepticism  in  my  mind  in  regard  to  the  accounts  of  other 
observers,  who  give  an  even  spermatogonial  number  for  forms  possessing  an  accessory  chromosome; 
and  if  this  be  well  founded  the  objection  urged  above  disappears.  I  shall  return  to  this  subject 
hereafter.  It  is  needless  to  say  that  had  I  been  acquainted  with  these  facts,  the  discussion  that 
follows  would  have  been  different. 


400  Edmund  B.  Wilson. 

nucleoli"  which  in  such  forms  as  Anasa  or  Alydus  conjugate  to 
form  the  small  central  chromosome  of  the  first  mitosis.  The  dif- 
ferences between  the  two  forms  have  already  been  pointed  out. 
Their  resemblances  are,  however,  no  less  obvious,  namely,  their 
usual  central  position  in  the  equatorial  plate,  small  size,  occasional 
persistence  as  chromosome-nucleoli  in  the  growth-period,  and 
their  late  conjugation.  This  comparison  finds  very  definite  support 
in  the  conditions  I  have  described  in  Nezara,  where  the  idio- 
chromosomes  are  of  equal  size  and  appear  as  two  equal  micro- 
chromosomes  in  the  spermatogonia.  From  the  analogy  of  other 
forms  it  is  very  probable  that  the  more  primitive  and  typical  form 
of  synapsis  is  that  between  chromosomes  of  like  size.  It  is  there- 
fore probable  that  such  a  condition  as  that  observed  in  Nezara 
is  a  less  modified  one  than  that  in  which  the  idiochromosomes  are 
unequal;  and  that  the  latter  condition  has  arisen  through  a  second- 
ary morphological  differentiation  of  two  chromosomes  that  were 
originally  of  equal  size,  and  perhaps  are  represented  by  the  two 
equal  microchromosomes  that  appear  in  the  spermatogonia  of 
such  Hemiptera  as  Anasa,  Alydus,  Syromastes  or  Protenor. 
This  comparison  involves  two  assumptions,  namely,  first  that  in 
case  of  the  idiochromosomes  the  final  conjugation  of  the  micro- 
chromosomes  has  been  postponed  from  the  prophases  of  the  first 
division  to  those  of  the  second;  and  secondly  that  a  reversal  in  the 
order  of  the  reduction-  and  equation-divisions  has  taken  place  in 
case  of  these  particular  chromosomes,  the  first  division  being  in 
case  of  the  Anasa-type  the  reduction-  and  in  case  of  the  idiochro- 
mosomes the  equation-division.  The  difficulty  apparently  involved 
by  the  second  assumption  is  less  serious  than  may  appear.  All 
the  facts  at  our  command  indicate  that  a  reduction-division 
is  the  necessary,  or  at  least  invariable,  sequel  to  a  foregoing 
conjugation;  and  if,  as  in  the  case  of  the  idiochromosomes,  the 
final  conjugation  is  deferred  to  the  second  division,  the  reduction- 
division  must  also  be  deferred.  The  univalent  idiochromosomes — 
as  is  shown  with  certainty  in  case  of  the  larger  one  in  Lygaeus— 
undergo  longitudinal  division  at  the  same  stage  of  the  growth- 
period  as  their  bivalent  companions  and  are  already  double 
at  the  time  of  the  first  mitosis.  There  is,  therefore,  no  difficulty 
in  the  way  of  assuming — indeed,  the  facts  seem  to  admit  of  no 
other  conclusion — that  this  is  the  equation-division. 

It  must  be  recognized,  however,  that  the  foregoing  comparison 


Studies  on  Chromosomes.  401 

wholly  fails  to  explain  the  origin  or  meaning  of  the  accessory 
chromosome,  nor  does  it  account  for  the  surprising  fact  (of  which 
the  phenomena  in  Brochymena  seem  to  leave  no  doubt)  that  two 
chromosomes  may  unite  in  synapsis,  subsequently  part  company 
so  as  to  divide  as  univalents  in  the  first  mitosis,  but  again  con- 
jugate to  form  a  bivalent  in  the  second  mitosis.  It  seems  likely 
that  further  comparative  study  of  this  phenomenon  may  throw 
important  light  on  the  general  mechanism  of  karyokinesis  and 
reduction. 

The  history  of  the  idiochromosomes  possesses  a  more  general 
interest  in  the  strong  support  that  it  lends  to  the  general  theory  of 
the  individuality  of  chromosomes,  to  the  specific  conclusions  of 
Montgomery  and  Sutton  in  regard  to  synapsis,  and  especially  to 
the  correlation  of  the  phenomena  of  reduction  with  those  of  Men- 
delian  inheritance  attempted  by  the  last-named  author  ('02,  '03). 
It  has  been  assumed  by  some  authors,  including  some  of  those  who 
have  accepted  Montgomery's  remarkable  conclusion  ('01,  i,  '04) 
that  corresponding  paternal  and  maternal  chromosomes  unite  in 
synapsis,  that  in  this  process  the  individuality  of  the  conjugating 
chromosomes  is  completely  lost — "Sie  vereinigen  sich  zu  einem 
einzigen  Zygosom,  aus  dem  erst  wieder  zwei  neue  Chromosomen 
hervorgehen."1  It  is  undoubtedly  true  that  frequently  all  visible 
traces  of  the  duality  of  the  bivalents  that  emerge  from  the  synapsis 
stage  are  for  a  time  lost;  and  as  Sutton  suggested  ('03,  p.  243),  such 
cases  as  those  of  first  crosses  that  breed  true — and  I  may  add, 
perhaps  also  those  in  which  blended  inheritance  or  weakening  of 
dominance  occurs — may  be  taken  to  indicate  that  a  permanent 
fusion,  or  intermixture  of  the  chromosome-substances,  may  really 
take  place.  But,  on  the  other  hand,  the  history  of  the  idiochro- 
mosomes in  cases  where  they  remain  separate  through  the  whole 
growth-period  leaves  not  the  least  doubt  that  as  far  as  these 
particular  chromosomes  are  concerned  the  same  two  that  unite  in 
synapsis  persist  as  distinct  individuals  to  be  afterward  separated 
by  the  reducing  division  and  assigned  to  different  germ-cells. 
This  preliminary  conjugation  and  subsequent  separation  ensures 
that  the  germ-cells  shall  be  "pure"  in  respect  to  these  particular 
chromosomes — /.  ^.,  that  both  shall  not  enter  the  same  spermat- 
ozoon— and  if  this  be  true  of  one  pair  of  the  conjugating  chromo- 


'Strasburger,  '04,  p.  26;  cf.  also  Bonnevie,  'o 


402  Edmund  B.  Wilson. 

somes  we  have  good  reason  to  conclude  that  it  may  be  true  of  all, 
as  Montgomery  has  urged  and  as  Sutton  has  so  cogently  argued, 
from  a  study  of  the  size-relations.  It  is  a  fair  working  hypothesis 
that  the  idiochromosomes  represent  a  pair  of  corresponding  or 
allelomorphic  qualities,  or  group  of  qualities,  that  are  respectively 
maternal  and  paternal,  as  Sutton,  building  on  the  basis  laid  by 
Montgomery  and  himself,  has  argued  for  the  chromosome-pairs 
in  general.  The  argument  of  Montgomery  and  Sutton  is  based, 
it  is  true,  on  the  fact  that  chromosomes  of  different  sizes  in  the 
spermatocytes  are  represented  by  symmetrical  chromosome-pairs 
of  corresponding  sizes  in  the  spermatogonia;  and  to  this  the  idio- 
chromosomes in  most  of  the  cases  described  form  an  exception  in 
being  unequal.  If  this  appears  to  be  a  difficulty  it  is  removed  by 
the  case  of  Nezara,  where  the  idiochromosomes  are  of  equal  size. 
Even  in  the  more  usual  case,  where  they  are  unequal,  symmetrical 
synapsis  takes  place  between  all  the  other  chromosome-pairs. 
If  the  theory  of  the  individuality  of  chromosomes  be  granted  no 
other  conclusion  seems  possible,  accordingly,  than  that  the  remain- 
ing two,  despite  their  size-difference,  are  respectively  the  paternal 
and  maternal  elements  of  the  remaining  pair;  and  if  Sutton's 
general  hypothesis  be  well  founded,  these  elements  may  be 
assumed  to  be  physiological  correlates  or  allelomorphs.  Their 
marked  difference  in  size  suggests  a  corresponding  qualitative 
differentiation,  and  this  inevitably  suggests  a  possible  connection 
between  them  and  the  sexual  differentiation.  The  visible  dimor- 
phism of  the  spermatid-nuclei  in  such  forms  as  Lygaeus,  Ccenus 
or  Podisus  shows  too  obvious  a  parallel  to  the  sexual  dimorphism 
of  the  germ-cells,  indicated  by  so  much  of  the  recent  work  on  sex- 
determination,  to  be  ignored;  while  in  Nezara,  where  no  visible 
dimorphism  exists,  the  spermatozoa  nevertheless  fall  into  two 
equal  groups  in  respect  to  the  previous  behavior  of  one  of  the 
chromosomes.  But  such  a  suggestion  as  to  the  possible  signifi- 
cance of  the  idiochromosomes  immediately  encounters  the  diffi- 
culty that  both  idiochromosomes  are  present  in  the  male  cells 
(spermatogonia,  and  spermatocytes),  just  as  McClung's  similar 
hypothesis  regarding  the  accessory  chromosome  is  confronted  with 
the  fact,  determined  by  Montgomery  and  Gross,  that  in  the  Hem- 
iptera  both  sexes  show  the  same  number  of  chromosomes. 
Whether  these  difficulties  can  be  met  by  assumptions  of  dominance 
and  the  like  remains  to  be  seen;  but  the  fact  should  be  recognized 


Studies  on  Chromosomes.  403 

that  as  far  as  the  Hemiptera  are  concerned  neither  the  suggestion 
I  have  made,  nor  the  hypothesis  of  McClung  has  at  present  any 
direct  support  in  observed  fact.1 

The  practical  interest  of  the  idiochromosomes  lies  in  the  very 
definite  basis  that  they  give  for  an  examination  of  the  question  by 
the  study  of  fertilization,  for  their  disparity  in  size  gives  us  the 
hope  of  determining  their  history  by  direct  observation.  There 
is  good  reason  to  believe  that  such  a  study  will  yield  interesting 
results. 

SUMMARY   OF    OBSERVATIONS. 

1.  In    Lygaeus    turcicus,  Coenus    delius,    Euschistus    fissilis, 
Euschistus  sp.,   Brochymena,  Nezara,  Trichopepla  and   Podisus 
spinosus  all  of  the  spermatids  receive  the  same  number  of  chromo- 
somes (half  the  spermatogonial  number),  and  no  accessory  chro- 
mosome is  present;  but  the  spermatozoa  nevertheless  consist  of 
two  groups,  equal  in  number,  which  differ  in  respect  to  one  of  the 
chromosomes,  which  may  conveniently  be  called  the  "idiochromo- 
some." 

2.  In   all  of  the  forms  named,   excepting  Nezara,   half  the 
spermatozoa  receive  a  larger,  and  half  a  smaller,  idiochromosome. 
In  Nezara  the  idiochromosomes  are  of  equal  size,  but  agree  in 
behavior  with  the  unequal  forms. 

3.  In  all  of  the  forms  the  idiochromosomes  remain  separate 
and  univalent  in  the  first  maturation-division,  while  the  other 
chromosomes  are  bivalent;  this  division  accordingly  shows  one 
more   than   half  the   spermatogonial   number   of  chromosomes. 
They  divide  separately  in  the  first  mitosis,  but  at  the  close  of  this 
division  their  products  conjugate  to  form  a  dyad,  which  in  all  the 
forms  save  Nezara  is  asymmetrical.     The  number  of  separate 

'The  discovery,  referred  to  in  a  preceding  foot-note,  that  the  spermatogonial  number  in  Anasa  is 
21  instead  of  22,  again  goes  far  to  set  aside  the  difficulties  here  urged.  Since  this  paper  was  sent  to 
press  I  have  also  learned  that  Dr.  N.  M.  Stevens  (by  whose  kind  permission  I  am  able  to  refer  to  her 
results)  has  independently  discovered  in  a  beetle,  Tenebrio,  a  pair  of  unequal  chromosomes  that  are 
somewhat  similar  to  the  idiochromosomes  in  Hemiptera  and  undergo  a  corresponding  distribution  to 
the  spermatozoa.  She  was  able  to  determine,  further,  the  significant  fact  that  the  small  chromosome  is 
present  in  the  somatic  cells  of  the  male  only,  while  in  those  of  the  female  it  is  represented  by  a 
larger  chromosome.  These  very  interesting  discoveries,  now  in  course  of  publication,  afford,  I  think, 
a  strong  support  to  the  suggestion  made  above;  and  when  considered  in  connection  with  the  com- 
parison I  have  drawn  between  the  idiochromosomes  and  the  accessory  show  that  McClung's  hypo- 
thesis may,  in  the  end,  prove  to  be  well  founded. 


404  Edmund  B.  Wilson. 

chromatin  elements  is  thus  reduced  to  one  half  the  spermatogonial 
number.  In  the  second  maturation-division  the  asymmetrical 
dyad  separates  into  its  two  unequal  constituents,  the  larger  one 
passing  to  one  pole  and  the  smaller  one  to  the  other  pole  of  the 
spindle,  while  the  other  dyads  divide  equally. 

4.  In    all    the    forms    excepting   Nezara    the    spermatogonia 
possess  but  one  microchromosome  (the  small  idiochromosome), 
while  in  Nezara  two  equal  microchromosomes  are  present  as  in 
forms  like  Anasa  which  possess  an  accessory  chromosome. 

5.  In  the  primary  synapsis  the  idiochromosomes  may  unite  to 
form  a  bivalent  body  or  may  remain  separate.     In  the  former  case 
the  bivalent  body  condenses  to  form  a  single  chromosome-nucleolus 
that    persists    throughout    the    whole    growth-period,    but    again 
separates  into  its  univalent  constituents  before  the  first  mitosis 
(directly  proved  in   Brochymena,  inferred  in  the  other  forms). 
If  the   idiochromosomes  fail  to  unite   in   the   primary  synapsis, 
they  remain  separate  through  the  growth-period  in  the  form  of 
chromosome-nucleoli.     In  either  case  the  idiochromosomes  divide 
separately  in  the  first  mitosis. 

6.  In  Lygaeus  the  large  idiochromosome  has  in  the  synaptic 
and  early  post-synaptic  periods  the  form  of  a  long  longitudinally 
split  thread  which  afterward  condenses  into  a  hollow  spheroidal 
chromosome-nucleolus. 

Zoological  Laboratory,  Columbia  University, 
May  5th,  1905. 

WORKS    CITED. 

BONNEVIE,    K.,  '05. — Das  Verhalten  des  Chromatins  in  den   Keimzellen  ente- 

roxenos  ostergreni.     Anat.  anz.,  xxvi,  13,  14,  15. 
GROSS,  J.,  '04. — Die  Spermatogenese  von  Syromastes  marginatus;     Zool.  Jahrb., 

Anat.  u.  Ontog.,  xx,  3. 
MONTGOMERY,   T.    H.,    '98. — The   Spermatogenesis   in    Pentatoma,    etc.     Zool. 

Jahrb.,  Anat.  u.  Ontog.,  xii. 
'01,  I. — A  Study  of  the  Chromosomes  of    the  Germ-cells  of  Metazoa. 

Trans.  Amer.  Phil.  Soc.,  xx. 
'01,  2. — Further  Studies  on  the  Chromosomes  of  the  Hemiptera  heterop- 

tera.     Proc.  Acad.  Nat.  Sci.,  Phil.,  March,  1901. 

'04. — Some    Observations    and    Considerations    upon    the    Maturation 
Phenomena  of  the  Germ-cells.     Biol.  Bull.,  vi,  3,  Feb. 


Studies  on  Chromosomes.  4°5 

PAULMIER,  F.  C.,  '99. — The  Spermatogenesis  of  Anasa  tristis.     Jour.  Morph., 

xv,  supplement. 
STRASBURGER,  E.,  '04.— Ueber  Reduktionsteilung.     Sitzber.  Kon.  Preuss.  Akad. 

Wiss.,  xviii,  24  Marz,  1904. 
SUTTON,  W.  S.,  '02. — On  the  Morphology  of  the  Chromosome  Group  in  Brachy- 

stola  magna.     Biol.  Bull.,  iv,  I. 

'03. — The  Chromosomes  in  Heredity.     Biol.  Bull.,  iv,  5. 
WILSON,  E.  B.,  '05. — Observations   on  the   Chromosomes  in  Hemiptera.     Rept. 

N.  Y.  Academy  of  Sciences,    May  8th,  1905;  Science,  xxi,  548, 

June  30. 


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STUDIES   ON   CHROMOSOMES 

II.     THE  PAIRED  MICROCHROMOSOMES,  IDIOCHRO 
MOSOMES  AND  HETEROTROPIC  CHRO- 
MOSOMES IN  HEMIPTERA 


By 

\r>  B.  WILSON 


RETURN  TO 

DIVISION  OF  GENETICS 

HILGARD  HALL 


REPRINTED   FROM 

THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY 

Volume   II 

No.  4 


BALTIMORE,  MD.,   U.  S.  A. 

November,    1905. 


STUDIES  ON  CHROMOSOMES. 

II.    THE  PAIRED   MICROCHROMOSOMES,  IDIOCHRO- 
MOSOMES  AND   HETEROTROPIC  CHRO- 
MOSOMES IN    HEMIPTERA.1 

BY 

EDMUND   B.  WILSON. 
WITH  4  FIGURES. 

In  investigating  the  physiological  significance  of  the  chro- 
mosomes and  their  individual  values  in  heredity,  it  is  important 
to  determine  as  accurately  as  possible  how  far  they  are  differen- 
tiated in  respect  to  individual  behavior,  and  to  ascertain  by  the 
comparative  study  of  different  forms  to  what  extent  the  chro- 
mosomes can  be  grouped  in  well-defined  classes.  The  work  of 
Henking,  Paulmier,  Montgomery,  Gross  and  Stevens  on  the  Hemip- 
tera  has  shown  that  this  group  is  peculiarly  favorable  for  such  a 
study;  and  I  believe  from  my  own  observation  that  no  group  of 
animals  has  thus  far  been  examined  that  offers  greater  advan- 
tages in  this  direction.2  But  although  the  general  results  obtained 
by  the  above-mentioned  observers  are  of  great  value  and  interest 
they  nevertheless  show  many  discordances  of  detail  that  stand  in 
the  way  of  a  consistent  general  interpretation  of  the  phenomena, 
while  some  of  Gross's  conclusions  are  a  stumbling  block  in  the 
way  of  the  whole  theory  of  the  individuality  of  the  chromosomes. 
For  this  reason  I  propose  in  this  paper  to  record  a  series  of  obser- 
vations that  I  hope  may  serve  to  clear  away  some  of  the  con- 
fusion that  now  exists  in  the  accounts  of  the  subject,  and  that 
open  the  way,  I  believe,  to  a  true  interpretation  of  the  "accessory 
chromosome"  and  its  relation  to  the  determination  of  sex. 

In  a  series  of   suggestive  papers  ('01,  '04,  '05)  Montgomery 

'Attention  is  called  to  the  Appendix  in  which  are  briefly  recorded  facts,  determined  by  later 
observations,  that  exactly  realize  the  theoretic  expectation  regarding  the  sexual  differences  of  the 
chromosome-groups,  stated  at  p.  539.  An  abstract  of  these  observations  was  published  in  the  issue  of 
Science  for  Oct.  20,  1905. 

2J  am  much  indebted  to  Mr.  Uhler's  kindness  in  identifying  many  of  the  species  examined. 


508  Edmund  B.  Wilson. 

has  endeavored  to  bring  together  under  the  name  of  "hetero- 
chromosomes"  two  classes  of  chromosomes  in  these  insects, 
namely,  the  "unpaired  heterochromosome"  ("accessory  chro- 
mosome" of  McClung)1  and  the  "paired  heterochromosomes" 
(or  "chromatin  nucleoli"),  which  differ  markedly  in  behavior 
from  the  other  chromosomes  during  the  maturation  process. 
Montgomery  gives  as  the  most  essential  characteristic  of  these 
chromosomes  "their  difference  in  behavior  from  the  other  chro- 
mosomes in  the  growth  period  of  the  spermatocytes  and  ovocytes, 
as  sometimes  during  the  rest  period  of  the  spermatogonia,  a  dif- 
ference which  appears  usually  to  consist  in  the  maintenance  of 
their  compact  structure  and  deep-staining  intensity,  so  that  while 
the  other  chromosomes  become  long  loops  or  even  compose  a 
reticulum,  these  do  not  undergo  any  such  changes  or  only  to 
slight  extent"  ('05,  p.  191).  "Thanks  to  this  peculiarity  they 
can  be  followed  with  extreme  certainty  from  generation  to  genera- 
tion, even  during  rest  stages;  and  so  are  splendid  evidence  for  the 
thesis  of  the  individuality  of  the  chromosomes"  ('04,  p.  146). 

The  study  of  these  chromosomes  has  led  Montgomery  to  some 
very  important  conclusions  regarding  synapsis  and  reduction  with 
which,  as  far  as  their  more  general  features  are  concerned,  I  am 
glad  to  find  my  own  results  in  substantial  agreement.  Considered 
more  in  detail,  however,  there  are  many  points  regarding  which 
I  think  Montgomery's  general  treatment  of  the  "heterochro- 
mosomes" requires  emendation. 

In  a  preceding  paper  (Wilson, '05)  the  fact  was  indicated  that  two 
types  of  "paired  heterochromosomes"  or  "chromatin  nucleoli" 
occur  in  Hemiptera.  The  first,  including  what  I  have  called  the 


'Since  there  is  no  reason  for  considering  the  "accessory  chromosome"  as  in  any  sense  accessory  to 
the  others,  it  appears  to  me  that  McClung's  term  might  well  be  abandoned  in  favor  of  a  less  com- 
promising one.  I  suggest  that  until  their  physiological  significance  is  positively  determined  chro- 
mosomes of  this  type  may  provisionally  be  called  heterot'op;c  chromosomes  (in  allusion  to  the  fact  that 
they  pass  to  one  pole  only  of  the  spindle  in  one  of  the  maturation-divisions)  in  contradistinction  to 
amphitropic  chromosomes,  the  products  of  which  pass  to  both  poles  in  both  divisions.  There  are  several 
objections  to  this  term,  one  of  which  is  that  the  "accessory''  chromosome  behaves  as  a  heterotropic 
body  in  only  one  of  the  divisions  (and  probably  in  one  sex  only).  Another  is  the  fact  that  the  members 
("chromatids")  of  every  chromosome-pair  are  heterotropic  in  the  reducing  division,  since  this  only 
separates  univalent  chromosomes  that  were  previously  in  synapsis;  but  if,  as  in  these  studies,  the  term 
"chromosome''  be  consistently  applied  to  each  coherent  chromatin-element  of  the  equatorial  plate, 
whatever  be  its  valence  or  mode  of  origin,  this  objection  is  perhaps  not  serious  enough  to  weigh  against 
the  convenience  of  the  term. 


Studies  on  Chromosomes.  5°9 

"idiochromosomes"  (which  occur  in  such  forms  as  Lygseus, 
Euschistus,  Coenus,  Brochymena,  etc.)  are  typically  unequal  in 
size,  and  differ  from  all  other  known  forms  of  chromosomes  in 
the  fact  that  their  union  in  synapsis  gives  rise  to  an  unequal  or 
asymmetrical  bivalent.  The  spermatogonial  groups  correspond- 
ingly show  but  one  small  chromosome,  since  the  larger  idio- 
chromosome  is  not  noticeably  smaller  than  the  ordinary  chromo- 
somes. The  second  type  includes  the  equal  paired  "chromatin 
nucleoli"  of  such  forms  as  Anasa,  Alydus,  Syromastes  or 
Archimerus.  Since  the  latter  are  almost  always  markedly 
smaller  than  the  others  they  may  conveniently  be  called  the 
paired  microchromosomes,  or  better,  in  order  to  avoid  all 
ambiguity,  simply  the  m-chromosomes;  and  these  are  distin- 
guishable in  the  spermatogonial  groups  as  an  equal  pair  of 
especially  small  chromosomes.  The  most  obvious  difference  of 
behavior  between  these  two  types,  so  far  as  is  now  known,  is  that 
the  idiochromosomes  divide  as  separate  univalents  in  the  first 
maturation-mitosis,  which  accordingly  always  shows  one  more 
than  half  the  spermatogonial  number  of  separate  chromatin 
elements,  while  the  m-chromosomes,  like  the  other  chromosomes, 
always  unite  to  form  a  bivalent  before  the  first  mitosis — which 
therefore  shows  the  same  number  as  in  the  second  division. 
Other  no  less  characteristic  differences  are  described  beyond. 
These  two  forms  are  not  yet  known  to  coexist  in  the  same  species; 
and,  as  a  rule,  forms  that  possess  the  idiochromosomes  do  not 
have  an  "accessory"  or  heterotropic  chromosome,  while  as  far 
as  now  known  such  a  chromosome  is  always  associated  with  the 
m-chromosomes. 

The  confusion  that  has  grown  out  of  the  failure  to  observe  these 
differences  arose  in  the  first  instance  from  two  conclusions — both  of 
which  I  shall  show  to  be  untenable — reached  by  Paulmier  in  his 
valuable,  and,  as  far  as  the  general  history  of  the  maturation- 
process  is  concerned,  very  accurate,  study  of  the  spermatogenesis 
of  Anasa  tristis  ('99),  and  was  increased  by  the  subsequent  efforts 
of  Montgomery  ('01,  '04,  '05)  to  reduce  the  behavior  of  the 
"chromatin  nucleoli"  to  a  uniform  scheme.  Paulmier,  who  was 
the  first  to  reexamine  the  history  of  the  "accessory"  chromosome 
since  its  discovery  by  Henking,  was  also  the  first  to  describe  the 
m-chromosomes  (in  Anasa)  as  two  very  small  chromosomes  of 
equal  size  in  the  spermatogonial  metaphase-groups.  These  two 


510  Edmund  B.  Wilson. 

small  chromosomes,  he  believed,  united  in  synapsis  to  form  a 
single  condensed  bivalent  chromosome-nucleolus  which  persisted 
throughout  the  growth-period  of  the  spermatocytes  and  later  gave 
rise  to  the  small  central  "tetrad"  of  the  first  maturation-mitosis. 
He  believed,  further,  that  after  an  equal  division  of  this  small 
"tetrad"  in  the  first  mitosis  each  of  its  products  passed  undivided 
to  one  pole  of  the  second  spermatocyte-spindle.  He  therefore 
compared  the  "small  tetrad"  (microchromosome-bivalent)  of 
Anasa  to  the  body,  first  discovered  by  Henking  in  Pyrrochoris,  and 
afterward  found  in  the  Orthoptera  and  some  other  insects  by 
McClung  and  others,  to  which  the  last-named  author  gave  the 
name  of  "accessory  chromosome."  In  identifying  the  chro- 
mosome-nucleolus of  the  growth-period  as  the  microchromosome- 
bivalent  Paulmier  has  been  followed  by  Montgomery  in  all  of 
his  papers  and  with  some  modifications  by  Gross  ('04)  in  his  recent 
study  of  Syromastes.  Paulmier's  conclusion  on  this  point  cannot, 
however,  be  sustained,  as  I  shall  try  to  show;  and  the  same  is  true 
of  his  identification  of  the  microchromosome-bivalent  as  the 
"accessory"  or  heterotropic  chromosome. 

I.  GENERAL  HISTORY  OF  THE  M-CHROMOSOMES  AND  THE  HET- 
EROTROPIC CHROMOSOME  DURING  THE  GROWTH-PERIOD  AND 
IN  THE  MATURATION-DIVISIONS. 

The  behavior  of  the  m-chromosomes  in  the  maturation-divisions 
may  conveniently  be  considered  first. 

Paulmier's  original  preparations,1  as  well  as  my  own  more  recent 
ones,  give  demonstrative  evidence  of  the  equal  division  of  the 
small  central  chromosome  in  both  maturation-mitoses,  and  the 
same  appears  no  less  clearly  in  Alydus  and  in  Archimerus,  pre- 
cisely as  has  been  shown  by  Montgomery  ('01)  in  Protenor  and 
by  Gross  ('04)  in  Syromastes.  I  was  long  since  led  to  suspect  an 
error  in  Paulmier's  conclusion  in  regard  to  this  point  from  the 
fact,  which  clearly  appears  in  his  own  figures,  that  the  "accessory" 
is  nearly  or  quite  as  large  as  the  other  chromosomes,  and  much 
larger  than  the  products  of  the  first  division  of  the  small  bivalent. 


JI  have  in  the  previous  paper  acknowledged  my  indebtedness  to  Dr.  Paulmier's  generosity  in  placing 
at  my  disposal  his  entire  series  of  preparations  of  Anasa  and  other  insects.  He  has  since  added  to  this 
indebtedness  by  sending  me  from  time  to  time  a  large  amount  of  valuable  living  material. 


Studies  on  Chromosomes.  511 

(Cf.  Paulmier's  Figs.  28,  34-36,  and  my  Fig.  2,  k-n.)  Both  in 
Anasa  and  in  Alydus  careful  search  among  longitudinal  sections 
of  the  second  division  shows  in  fact  in  the  clearest  manner,  that 
the  "small  dyad"  divides  into  equal  halves,  so  that  each  of  the 
spermatids  received  one  of  its  products  (Figs.  I,  i-m\  2,  m,  n). 
The  heterotropic  chromosome  is  a  much  larger  body,  as  shown 
by  the  figures,  in  Anasa  fully  equal  in  size  to  some  of  the  larger 
single  chromosomes  of  the  anaphases  of  the  second  division. 
Paulmier's  failure  to  observe  the  second  division  of  the  small 
bivalent  is  easily  explained  by  the  difficulty  of  observing  this  body 
owing  to  its  usually  central  or  subcentral  position,  and  the  mistake 
was  a  very  natural  one  at  the  time  his  paper  was  written.  Had  he 
examined  Alydus  where  there  are  but  seven  chromosomes,  which 
show  marked  and  constant  size-differences,  he  could  not  have 
failed  to  observe  this  division. 

We  have  now  to  examine  a  second  and  more  difficult  point, 
namely,  the  nature  of  the  condensed  nucleolus-like  body 
(chromosome-nucleolus)  of  the  growth-period,  which  so  closely 
simulates  the  heterotropic  chromosome  of  the  Orthoptera  at  the 
corresponding  period.  I  have  always  doubted  Paulmier's  and 
Montgomery's  conclusion  that  this  body  is  the  microchromosome- 
bivalent,  from  the  fact,  clearly  shown  in  the  figures  of  both  these 
authors,  that  the  chromosome-nucleolus  of  the  synaptic  and 
growth-periods  is  always  larger,  and  in  some  species  very  much 
larger  (e.  g.,  in  Alydus)1  than  the  two  spermatogonial  micro- 
chromosomes  taken  together,  or  than  the  small  central  bivalent 
to  which  it  was  assumed  to  give  rise.  (Cf.  Paulmier's  Figs.  16-21, 
with  26,  28.)  This  fact  did  not  escape  Montgomery's  attention, 
but  he  explained  it  as  due  to  an  increase  of  volume  on  the  part 
of  the  chromatin-nucleolus  in  the  early  growth-period  and  a 
corresponding  decrease  in  the  late  growth-period  or  in  the  pro- 
phases  of  the  first  division  ('01,  p.  203).  This  explanation  was, 
however,  not  supported  by  any  sufficient  evidence;2  and  the  only 
detailed  evidence  on  this  point  has  been  brought  forward  by 
Gross  ('04)  in  the  case  of  Syromastes.  This  observer,  however, 
while  apparently  confirming  Paulmier  and  Montgomery  as  to 


lCf.  Montgomery,  '01,  Figs.  96-98. 

^Montgomery's  study  of  the  facts  in  Euschistus  ('98)  is  not  in  point,  since  he  was  here  undoubtedly 
dealing  with  the  idiochromosomes  and  not  with  the  w-chromosomes. 


512  '  Edmund  B.  Wilson. 

the  fate  of  the  chromosome-nucleolus,  differs  entirely  from  them 
in  regard  to  its  origin,  concluding  that  it  is  derived  from  two  of 
the  larger  spermatogonial  chromosomes.  In  the  attempt  to 
reconcile  these  contradictory  results  (with  both  of  which  my  own 
are  in  disagreement)  he  is  led  to  some  speculative  conclusions  that 
I  think  must  be  regarded  as  highly  improbable.1 

A  careful  study  of  all  the  intermediate  stages,  not  only  in  Anasa, 
but  also  in  Alydus,  Archimerus,  and  Chariesterus  gives  in  point 
of  fact,  evidence  that  I  believe  is  quite  decisive,  that  the  small 
central  bivalent  is  not  derived  from  the  large  chromosome-nucleolus 
of  the  growth-period,  and  that  the  latter  is  nothing  other  than  the 
accessory  or  heterotropic  chromosome,  precisely  as  in  the  Orthoptera. 
To  the  differences  between  the  idiochromosomes  and  the  m-chro- 
mosomes  already  stated  may  therefore  be  added  the  fact  that  the 
former,  like  the  heterotropic  chromosome,  may  form  a  single 
chromosome-nucleolus  during  the  growth-period,  while  this  is 
not  the  case,  in  the  forms  I  have  studied,  with  the  m-chromosomes. 
It  may  seem  strange  that  Montgomery,  after  accurately  tracing 
the  history  of  the  heterotropic  chromosome  ("chromosome  *") 
in  Protenor  and  showing  its  complete  independence  of  the  "chro- 
matin-nucleoli"  (m-chromosomes)  was  not  led  to  suspect  a 
similar  relation  in  the  other  forms.  That  he  apparently  did  not 
do  so  was  doubtless  due  to  his  having  failed  to  distinguish  between 
the  m-chromosomes  and  the  idiochromosomes,  which  latter  bodies 
he  correctly  identified  (in'Euschistus,  etc.)  as  the  bivalent  chromo- 
some-nucleolus (or  two  separate  univalents)  of  the  growth-period. 

The  entire  independence  of  the  large  chromosome-nucleolus 
and  the  m-chromosomes  is  most  obvious  in  Alydus  and  Archi- 
merus, partly  because  in  both  these  forms  the  heterotropic 
chromosome  is  at  every  period  recognizable  by  its  characteristic 
size,  partly  because — in  Alydus  certainly,  and  I  believe  also  in 
Archimerus — the  m-chromosomes  frequently  assume  a  compact 
condensed  form  at  a  much  earlier  period  than  in  Anasa;  they  can 
therefore  be  recognized  in  addition  to  the  heterotropic  chromosome, 
throughout  the  latter  part  of  the  growth-period,  at  a  time  when 
the  larger  chromosomes  are  still  in  the  pale,  vague  condition 
characteristic  of  so  many  of  the  Hemiptera  at  this  period. 

In  Alydus  pilosulus  the  first  mitosis  invariably  shows  seven 


'Gross  ('04)  pp.  481, 482. 


Studies  on  Chromosomes .  5*3 

bivalent  chromosomes,  which  show  very  marked  and  characteristic 
size-differences  (Fig. 'i,  c-g,  £).  There  are  always  (i)  a  largest 
chromosome  or  macrochromosome,  which  is  frequently  quad- 
ripartite; (2)  a  second  largest;  (3)  three  slightly  smaller  ones  of 
nearly  equal  size;  (4)  a  fourth,  considerably  smaller  than  the  last; 
and  finally  (5)  the  smallest  or  michrochromosome-bivalent. 
These  show  a  characteristic  grouping,  the  five  larger  ones  forming 
an  irregular  ring  with  the  small  bivalent  ("chromatin  nucleolus") 
at  its  center,  while  the  next  smallest  lies  more  or  less  at  one  side 
of  the  ring  (Fig.  i,  g}.  In  the  first  division  all  these  chromosomes 
are  equally  halved  (Fig.  i,  /).  In  the  second  all  are  again  halved 
with  the  exception  of  the  second  smallest  which  passes  undivided 
to  one  pole  of  the  spindle  (Fig.  i,  /-o).  The  size-relations  leave 
not  the  least  doubt  that  this  chromosome  is  derived  from  the  one 
of  corresponding  size  in  the  first  division — *.  e.,  the  odd  or  eccen- 
tric one — and  the  latter  accordingly  is  to  be  identified  as  the 
"accessory"  or  heterotropic  chromosome.  In  the  first  division 
this  chromosome  sometimes  shows  a  quadripartite  form  (as  was 
described  by  Paulmier  in  Anasa)  sometimes  a  dumbbell-shaped 
or  dyad-like  form.  In  the  second  it  is  usually  unconstricted  and 
often  curved  (Fig.  I,  z,  ra,  w),  sometimes  into  a  U-shape  so  as 
almost  to  appear  double  (Fig.  I,  o). 

A  study  of  the  growth-period  shows  that  the  heterotropic 
chromosome  may  be  traced  uninterruptedly  backward  from  the 
metaphase  of  the  first  division  to  the  contraction-phase  of  the 
synaptic  period,  being  always  in  the  form  of  a  condensed  chro- 
mosome-nucleolus,  which  in  the  early  growth-period  is  attached 
to  a  large,  pale  plasmosome,  from  which  it  afterwards  separates. 
It  is  impossible  to  mistake  this  chromosome,  owing  to  the  fact 
that  its  characteristic  size  does  not  noticeably  change  except  that 
it  becomes  slightly  larger  as  the  growth-period  advances  (probably 
owing  to  the  presence  of  a  central  cavity),  again  becoming  slightly 
smaller  as  the  general  condensation  takes  place.  (Cf.  Fig.  i, 
a-c.~)  In  the  contraction-phase  (Fig.  I,  #)  and  in  the  early  post- 
synaptic  spireme  the  m-chromosomes  are  not  visible,  but  as  the 
larger  chromosomes  assume  the  peculiar  pale,  ragged,  clumped 
condition,  characteristic  of  the  middle  and  late  growth-periods, 
the  m-chromosomes  frequently  come  into  view,  in  the  form  of 
two  compact,  intensely-staining  bodies,  that  may  occupy  any 
relative  position  (Fig.  I,  &).  The  period  at  which  these  bodies 


5H  Edmund  B.  Wilson. 


FIGURE  I. 

Alydus  pilosulus. — a,  Contraction-phase  of  synaptic  period,  "accessory"  (h)  in  the  form  of  a  con- 
densed chromosome-nucleolus  attached  to  a  large  plasmosome  (/>);  b,  spermatocyte-nucleus,  middle 
growth-period,  showing  large  diffused  chromosomes — "accessory"  still  attached  to  the  plasmosome — 
and  the  two  condensed  m-chromosomes  on  opposite  sides  of  the  nucleus;  c,  early  proph?se  of  first 
division,  showing  all  of  the  chromosomes,  the  larger  ones  condensing;  d,  late  prophase,  showing 
"accessory''  (h)  and  the  two  w-chromosomes  still  separate;  e,  slightly  later  prophase,  showing  all 
of  the  chromosomes;  /,  initial  anaphase,  first  division,  the  w-chromosomes  separating;  g,  polar  view  of 
metaphase-group,  first  division;  h,  polar  view  of  metaphase-figure,  second  division;  /',  j,  initial 
anaphases,  second  division;  k,  spermatogonial  metaphase-group;  /,  m,  n,  o,  anaphases  of  second 
division. 


Studies  on  Chromosomes. 


5'5 


FIGURE  i.1 


]Thc  figures  are  all  drawn  to  the  same  scale  as  those  of  the  preceding  study. 


516  Edmund  B.  Wilson. 

condense  into  the  compact  form  appears  to  vary  considerably, 
for  they  cannot  always  be  distinguished  until  the  later  growth- 
period,  and  it  should  be  noted  that  during  the  pale  period  the 
nuclei  often  show  a  variable  number  of  smaller  deeply-staining 
granules.  I  believe,  however,  that  there  can  be  no  doubt  as  to 
the  nature  of  the  two  larger  bodies  on  account  of  their  great  con- 
stancy, their  size,  and  the  completeness  of  the  series  that  connects 
the  earlier  with  the  later  conditions  (such  as  is  shown  in  Fig.  I,  <:), 
where  no  doubt  of  their  nature  can  exist.  The  persistence  of  the 
larger  chromosome-nucleolus  ("accessory")  throughout  all  these 
stages  without  any  considerable  change  renders  it  manifestly 
impossible  that  it  should  give  rise  to  the  ra-chromosome  bivalent, 
either  directly  as  assumed  by  Paulmier  and  Montgomery,  or  by 
division  into  two  univalents  that  subsequently  conjugate,  as 
described  by  Gross  in  Syromastes. 

In  the  early  prophases  the  larger  chromosomes  resume  their 
staining  capacity  and  condense  into  characteristic  cross-forms 
(Fig.  i,  <:),  and  finally  into  compact  quadripartite  tetrads  or  bipar- 
tite bodies.  At  this  time  the  heterotropic  chromosome  assumes 
a  dumbbell  or  quadripartite  shape,  and  the  w-chromosomes, 
which  are  still  quite  separate  and  may  even  lie  on  opposite  sides 
of  the  nucleus,  also  frequently  become  bipartite.  The  nucleus  now 
contains,  accordingly,  eight  separate  chromatin-elements,  one  more 
than  the  number  of  bivalents  in  the  first  mitosis,  as  is  also  the  case 
in  Archimerus  and  Anasa,  as  described  beyond.  As  the  spindle 
forms  the  two  microchromosomes  lose  their  bipartite  shape, 
approach  each  other,  and  in  the  stage  just  preceding  the  metaphase 
finally  conjugate  to  form  the  small  bivalent  chromosome  at  the 
center  of  the  group.  Without  fusing,  the  two  halves  are  then 
immediately  separated,  the  division  always  taking  place  more 
rapidly  than  in  the  case  of  the  larger  chromosomes  (Fig.  I,  /). 

It  is  clear  to  demonstration  accordingly,  that  in  Alydus  the  small 
central  bivalent  does  not  arise  from  the  large  chromosome-nucleolus 
of  the  growth-period,  but  is  formed  by  the  late  conjugation  of  two 
separate  microchromosomes  that  have  no  genetic  connection  with 
that  body.  The  same  fact  is  shown  no  less  clearly  in  Archimerus 
calcarator  (which  shows  eight  chromosomes  in  the  first  mitosis), 
where  the  m-chromosomes,  and  the  corresponding  bivalent,  are 
of  extraordinary  minuteness  and  are  so  much  smaller  than  the  acces- 
sory that  they  could  not  possibly  be  confused  with  the  latter  (Fig.  3). 


Studies  on  Chromosomes.  517 

I  believe  that  in  this  form,  too,  the  m-chromosomes  are  fre- 
quently recognizable  as  condensed  separate  bodies  in  the  growth- 
period;  but  owing  to  their  minute  size  it  is  difficult  to  be  sure  of 
this.  In  any  case,  in  the  period  just  before  the  disappearance  of 
the  nuclear  membrane  they  are  quite  distinct  from  the  "acces- 
sory," which  is,  as  in  Alydus,  immediately  recognizable  by  its 
size  (Fig.  3,  g).  From  this  period,  as  in  Alydus,  the  latter  body 
may  be  traced  continuously  backward  into  the  growth-period. 

The  foregoing  facts,  observed  in  Alydus  and  Archimerus  are 
in  close  agreement  with  Montgomery's  results  on  Protenor, 
differing  only  in  that  the  condensation  of  the  m-chromosomes 
takes  place  somewhat  later.1  In  Anasa  the  condensation  of  these 
chromosomes  from  the  diffused  condition  takes  place  still  later; 
and  this,  combined  with  the  fact  that  the  "accessory"  cannot  be 
certainly  distinguished  from  the  other  larger  chromosomes  by  its 
size,  renders  the  question  more  difficult  of  solution  than  in  Alydus, 
though  I  believe  the  result  is  equally  decisive.  In  Anasa,  as  in 
Alydus  or  Archimerus,  the  small  central  bivalent  of  the  first 
equatorial  plate  is  formed  by  a  very  late  conjugation  of  two 
separate  microchromosomes  that  only  come  together  as  the  spindle 
forms,  precisely  as  Gross  describes  in  Syromastes.  Of  this  fact 
no  doubt  is  left  by  the  study  of  a  large  number  of  preparations 
that  show  every  stage  of  the  process,  step  by  step.  In  the  late 
prophases,  just  before  the  nuclear  membrane  disappears,  the  nuclei 
invariably  show  twelve  separate,  condensed,  intensely-staining 
chromatin-elements  (one  more  than  the  number  of  chromosomes 
in  the  first  mitosis)  in  addition  to  one  or  more  pale  rounded 
plasmosomes  with  which  the  chromosomes  cannot  for  a  moment 
be  confused.  Ten  of  these  are  larger  bivalents  which  have  the 
form  of  quadripartite  tetrads  or  dumbbell-shaped  bodies.  The 
remaining  two  are  much  smaller  bodies,  irregularly  ovoidal  or 
frequently  more  or  less  distinctly  bipartite  (m,  Fig.  2,  ^,  /);  they 
may  occupy  any  relative  position.  As  the  spindle  forms,  the 
microchromosomes  lose  their  bipartite  form,  assume  an  evenly 
rounded  ovoidal  shape,  and  conjugate  at  the  center  of  the  equa- 
torial plate  to  form  a  small  dyad-shaped  bivalent  (Fig.  2,  £-*)• 
Without  fusion  the  two  halves  are  then  immediately  drawn  apart 


'In  Alydus  pilosulus  this  author  believed  the  /n-chromosomes,  as  usual,  to   be  derived   from   the 
large  chromosome-nucleolus. 


5i8 


Edmund  B.  Wilson. 


FIGURE  z. 

Anasa  tristis. — a, Contraction-phase  of  synaptic  period, showing  "accessory"  (h~)  and  plasmosome  (p); 
b,  spermatocyte-nucleus,  late  groarth-period,  beginning  of  the  condensation,  showing  "accessory''  (A) 
and  the  m-chromosomes  (m);  c,  a  slightly  later  stage  than  the  last;  d,  later  stage,  immediately  before 
the  final  condensation,  from  a  long-extracted  preparation;  e,  /,  two  sections  of  one  nucleus,  show- 
ing all  of  the  twelve  chromosomes  immediately  before  the  disappearance  of  the  nuclear  membrane; 
g,  view  of  one  pole  of  the  late  prophase  just  after  disappearance  of  the  nuclear  membrane,  the  m-chro- 
mosomes  still  wide  apart;  h,  early  metaphase-group  in  side  view,  showing  approach  of  the  m-chromo- 
somes;  *,  four  chromosomes  from  the  metaphase,  conjugation  of  the  m-chromosomes  to  form  the  small 
central  bivalent;  j,  early  anaphase,  separation  of  the  m-chromosomes,  "accessory''  at  the  left;  k,  polar 
view  of  metaphase-group,  first  division;  /,  polar  view  of  metaphase-group,  second  division;  m,  n, 
anaphases  of  second  division,  showing  division  of  m-chromosomes  and  the  undivided  heterotropic 
chromosome;  o,  p,  spermatogonial  metaphase-groups  drawn  as  carefully  as  possible  to  show  sizes  of 
the  chromosomes. 


Studies  on  Chromosomes. 


5*9 


520  Edmund  B.  Wilson. 

in  the  initial  anaphase,  always  separating  in  advance  of  the 
larger  chromosome-halves  (Fig.  2,  /).  It  is  not  possible  in  the 
prophases  just  described  to  identify  the  heterotropic  chromosome; 
but  from  the  analogy  of  Alydus,  Syromastes  and  Archimerus  it 
may  be  assumed  with  great  probability  that  it  is  the  "odd  or 
eccentric  chromosome  which  in  the  metaphase-group  lies  outside 
the  principal  ring  (Fig.  2,  k). 

During  the  growth-period,  as  Paulmier  described,  the  chromo- 
somes, with  the  exception  of  the  single  conspicuous  chromosome- 
nucleolus,  remain  in  a  loose,  diffused,  lightly-staining  condition 
from  the  post-synaptic  spireme  stage  until  the  condensation  of  the 
tetrads  begins;  and  until  the  end  of  this  period  the  m-chromo- 
somes  cannot  be  distinguished.  Throughout  this  whole  period 
the  chromosome-nucleolus  is  distinctly  visible;  and  it  may  at 
every  period,  even  in  hematoxylin  preparations,  if  long  extracted, 
be  at  once  distinguished  from  the  true  nucleolus  or  plasmosome 
(as  is  shown  in  Paulmier's  figures),  since  the  former  stains  intensely 
black,  the  latter  pale  blue  or  in  double-stained  preparations,  pale 
red  or  yellow.  In  the  contraction-phase  of  the  synaptic  period 
it  is  more  or  less  elongated,  ovoidal,  or  sometimes  slightly  con- 
stricted in  the  middle  (Fig.  2,  a).  In  the  late  post-synaptic 
period,  at  a  time  when  the  other  chromosomes  are  beginning  to 
shorten  and  to  give  rise  to  the  characteristic  double  cross-figures 
and  V-figures  it  is  usually  more  or  less  elongated,  the  transverse 
constriction  is  less  obvious  or  disappears  from  view,  and  the  body 
often  shows  faintly  but  distinctly  a  longitudinal  split.  (Cf.  Paul- 
mier, Fig.  22.)  Slightly  later,  as  the  other  chromosomes  continue 
to  shorten  and  thicken,  the  chromosome-nucleolus  also  shortens 
and  thickens,  often  assuming  a  spheroidal  form  in  which  a  central 
cavity  may  sometimes  be  seen.  As  the  remaining  chromosomes 
condense  to  form  the  tetrads  it  again  alters  its  shape,  often  becom- 
ing bipartite  (Fig.  2,  £-</),  but  sometimes  showing  a  more  or  less 
distinctly  quadripartite  form  as  described  by  Paulmier  (e.  g.y  in  his 
Figs.  23,  24).  It  now  becomes  indistinguishable  from  the  other 
larger  chromosomes,  since  the  latter  have  also  condensed  into 
similar  tetrads  or  dyad-like  forms,  but  the  two  m-chromosomes  are 
immediately  recognizable  by  their  small  size.  It  might  therefore 
be  supposed  that  the  chromosome-nucleolus  has  divided  to  form 
the  two  microchromosomes,  as  Gross  believed  to  be  the  case  in 
Syromastes.  The  stage  that  immediately  precedes  this  gives, 


Studies  on  Chromosomes.  521 

however,  conclusive  evidence  that  such  is  not  the  case.  In  this 
stage  (corresponding  to  Paulmier's  Figs.  22,  23)  the  chromosome- 
nucleolus  is  still  unmistakably  recognizable  by  its  compact  and 
rounded  appearance,  while  the  other  chromosomes,  including  the 
two  microckromosomes  are  still  in  the  form  of  paler  and  more 
diffuse  bodies.  The  ra-chromosomes  at  this  period  (one  of 
them  is  clearly  shown  in  Paulmier's  Fig.  24)  appear  as  short, 
more  or  less  ragged,  paler,  irregular  rods  that  give  the  appearance 
of  being  longitudinally  split  (Fig.  2,  b-d}.  Some  of  the  cysts 
at  this  period  show  every  stage  in  the  condensation  of  these  two 
small  diffused  chromosomes  to  form  the  two  small,  dyad-like  micro- 
chromosomes  that  conjugate  to  form  the  small  central  bivalent. 
I  have  studied  numerous  nuclei  in  these  stages  with  great  care, 
and  believe  that  they  remove  every  doubt  that  the  two  micro- 
chromosomes  that  conjugate  to  form  the  small  central  bivalent  in 
Anasa  arise  neither  from  separate  small  condensed  bodies,  as  in 
Protenor  or  often  in  Alydus,  nor  from  the  single  large  chromosome- 
nucleolus  as  assumed  by  Paulmier,  Montgomery  and  Gross,  but 
from  diffused  masses  similar  to  the  larger  ordinary  chromosomes 
during  the  greater  part  of  the  growth-period.  The  same  fact  may 
be  seen  in  Chariesterus,  though  I  have  not  in  this  case  so  complete 
a  series  of  stages.  The  chromosome-nucleolus  must  therefore 
give  rise  to  one  of  the  larger  chromosomes ;  and  the  exact  agreement 
of  Anasa  with  Alydus  and  Archimerus,  save  in  the  one  point  of 
the  later  condensation  of  the  microchromosomes  in  the  former 
form,  justifies  the  confident  conclusion  that  in  Anasa  the  chro- 
mosome-nucleolus is  the  "accessory"  or  heterotropic  chromosome. 
Anasa,  Alydus,  Chariesterus,  and  Archimerus  thus  fall  in  line 
with  the  facts  observed  in  the  Orthoptera,  and  I  believe  the  same 
will  prove  to  be  the  case  with  other  Hemiptera  in  which  an  "odd," 
"accessory"  or  heterotropic  chromosome  occurs.1  This  result, 
which  is  wholly  at  variance  with  the  accounts  of  previous 
observers,  forms  the  first  step  in  clearing  away  the  confusion 
that  has  hitherto  stood  in  the  way  of  a  consistent  general  inter- 
pretation of  the  heterotropic  chromosome. 


VI  cannot  at  present  offer  a  definite  explanation  of  the  divergence  between  this  result  and  that  reached 
by  Gross  in  Syromastes.  Without  questioning  the  accuracy  of  his  figures,  I  feel  confident,  in  view  of 
what  I  have  seen  in  so  many  other  forms,  that  further  examination  of  this  genus  will  give  a  different 
result,  both  on  this  point  and  on  a  number  of  others. 


522  Edmund  B.  Wilson. 

2.       RELATION    OF    THE    CHROMOSOME-NUCLEOLUS    TO    THE    SPER- 
MATOGONIAL    CHROMOSOMES. 

In  view  of  the  foregoing  conclusion  it  will  readily  be  admitted 
that  a  derivation  of  the  chromosome-nucleolus  from  the  two 
spermatogonial  microchromosomes  is  a  priori  highly  improbable; 
and  in  point  of  fact,  all  the  actual  observations  not  only  of  myself, 
but  also,  I  believe,  of  Paulmier  and  Montgomery,  are  opposed 
to  such  a  conclusion. 

This  question  has  been  complicated  in  a  most  unfortunate  way 
by  errors  in  counting  the  spermatogonial  chromosomes.  It  was 
natural  that  the  earlier  observers  should  have  expected  to  find  an 
even  number  of  chromosomes  in  the  spermatogonial  divisions; 
and  the  number  is  in  point  of  fact  an  even  one  in  all  the  forms 
that  possess  the  idiochromosomes,  as  I  have  shown  in  the  first 
of  these  studies.  Regarding  the  forms  that  possess  an  accessory 
or  heterotropic  chromosome  the  existing  accounts  are,  however, 
in  conflict  in  giving  sometimes  an  even  number  (Anasa,  t.  Paul- 
mier and  Montgomery,  Syromastes,  /.  Gross,  Alydus  pilosulus,  t. 
Montgomery),  and  sometimes  an  odd  one  (Protenor,  Harmostes, 
(Edancala,  Alydus  eurinus,  t.  Montgomery).  A  similar  difference 
occurs  in  the  existing  accounts  of  the  spermatogonia  in  Orthoptera, 
some  of  which  are  described  as  showing  an  even  number  and  some 
an  odd.  This  contradiction  has  enormously  increased  the  com- 
plication of  the  subject;  for  it  has  necessarily  involved  the  view 
that  in  cases  showing  an  even  number  the  heterotropic  chromo- 
some is  a  bivalent  body,  formed  by  the  synapsis  of  two  of  the 
spermatogonial  chromosomes;  and  this,  in  turn,  very  naturally 
led  Montgomery  ('04,  '05,  etc.)  to  the  further  conclusion  that  in 
cases  showing  an  odd  number  one  of  the  chromosomes  (presum- 
ably the  "accessory")  is  already  bivalent  in  the  spermatogonia. 

I  myself  had  at  first  no  doubt  of  the  correctness  of  both  these 
interpretations,  and  my  faith  was  not  shaken  even  after  the  dis- 
covery that  the  number  is  13  in  Alydus  pilosulus  (Fig.  I,  A), 
15  in  Archimerus  (Fig.  3,  /),  and  21  in  "Chariesterus."1  When, 
however,  demonstrative  evidence  was  obtained  that  even  in  Anasa 
— in  opposition  to  the  concordant  results  of  Paulmier  and  Mont- 
gomery on  Anasa  and  those  of  Gross  on  the  related  form  Syro- 


JThe  indentification  of  this  form  (from  Paulmier's  material)  is  doubtful. 


Studies  on  Chromosomes.  523 

mastes — the  number  is  21  instead  of  22  (Fig.  2,  o,  p)  I  confess 
that  surprise  at  this  result  was  followed  by  skepticism  regarding 
all  of  the  accounts  asserting  the  occurrence  of  an  even  number  in 
other  forms.  This  result,  which  was  totally  unexpected  to  me, 
rests  on  the  study  of  a  large  number  of  division-figures  exactly  in 
the  metaphase,  many  of  which  are  of  almost  schematic  clearness. 
Of  these,  twenty-five  (selected  from  six  testes  from  different  indi- 
viduals, including  both  adults  and  larval  forms)  were  drawn  with 
the  camera,  chromosome  by  chromosome,  and  subsequently 
counted.  Without  one  exception  these  drawings  show  exactly 
twenty-one  chromosomes;  it  is  therefore  out  of  the  question  that 
my  result  (worked  out  on  Paulmier's  original  preparations)  can  be 
due  to  an  accidental  displacement  of  one  of  the  chromosomes  in 
the  process  of  sectioning,  or  to  other  similar  sources  of  error. 
I  believe  the  error  of  previous  observers  on  this  point  is  owing  to 
the  fact  that  one  or  more  of  the  chromosomes  sometimes  show  a 
more  or  less  obvious  constriction  near  the  middle,  and  the  larger 
ones  are  not  infrequently  curved — sometimes  almost  into  a 
U-shape — so  that  one  might  readily  be  mistaken  for  two. 

Quite  in  harmony  with  this  result  is  the  fact  that  in  Anasa  the 
metaphase  groups  always  show  not  two  but  three  chromosomes 
that  are  distinctly  larger  than  the  others,1  one  of  these  being 
obviously  without  a  mate  of  like  size,  while  all  the  others  may  be 
symmetrically  paired,  two  by  two,  as  a  study  of  Fig.  2,  o,  p,  will 
show.  It  is  obvious  therefore  that  the  heterotropic  chromosome, 
and  hence  the  chromosome-nucleolus  of  the  growth-period,  must 
be  compared  with  one,  not  two,  of  the  spermatogonial  chromo- 
somes. 

In  Alydus  the  heterotropic  chromosome  appears  in  the  con- 
traction phase  of  the  synaptic  period  as  an  ovoidal  single  body, 
always  attached  to  one  side  of  a  large  plasmosome  and  imme- 
diately distinguishable  from  the  latter  by  its  different  staining- 
reaction  (Fig.  I,  a).  Comparison  of  this  figure  with  that  of  the 
spermatogonial  chromosomes  (Fig.  I,  A),  shows  that  the  hetero- 
tropic chromosome  at  this  period  is  much  larger  than  the  two 
spermatogonial  microchromosomes  united.  In  the  spermato- 
gonial equatorial  plates  of  Alydus  or  Archimerus  it  is  not  possible 


'I  regret  to  find  myself  here  again  in  disagreement  with   Montgomery,  who  finds  only  two   large 
spermatogonial  chromosomes  in  Anasa  ('04,  p.  151,  Fig.  16). 


524  Edmund  B.  Wilson. 

positively  to  identify  the  heterotropic  chromosome  by  its  size; 
though  it  is  evidently  not  one  of  the  largest  ones,  since  the  latter 
form  a  symmetrical  pair  (Fig.  I,  £)  which  doubtless  unite  to  form 
the  single  macrochromosome  of  the  spermatocyte-divisions  (in 
accordance  with  Montgomery's  account  of  several  other  forms). 
In  Anasa,  however,  it  may  be  regarded  as  highly  probable  that 
the  heterotropic  chromosome  is  one  of  the  largest  three  chro- 
mosomes, the  remaining  two  of  which  pair  as  usual  to  form  the 
spermatocyte  macrochromosome-bivalent  (Fig.  2,  o,  /?).  This  is 
confirmed  by  comparison  with  the  chromosome-nucleolus  at  the 
synaptic  contraction-period  (Fig.  2,  a).  At  this  time  it  varies 
considerably  in  form,  but  is  always  more  or  less  elongate,  often 
ovoidal,  sometimes  almost  rod-shaped,  and  sometimes  more  or 
less  distinctly  constricted  in  the  middle;  it  rarely  appears  to  be 
composed  of  two  symmetrical  halves  (described  by  Gross  as  the 
typical  condition  in  Syromastes.)  It  is  rarely  attached  to  a 
plasmosome,  the  latter  body,  when  present,  being  usually  separate 
(as  in  Fig.  2,  «). 

The  discrepancy  in  size  between  the  chromosome-nucleolus 
and  the  spermatogonial  microchromosomes  is  here  still  greater 
than  in  Alydus.  On  the  other  hand,  as  a  comparison  of  the 
figures  will  show,  the  chromosome-nucleolus  of  this  period  is  of 
very  nearly  the  same  volume  as  one  of  the  largest  three  spermat- 
ogonial chromosomes.  All  the  facts  therefore  point  to  the  con- 
clusion that  one  of  the  latter  is  the  heterotropic  chromosome, 
and  that  it  persists  throughout  the  growth-period  as  the  chromo- 
some-nucleolus, precisely  as  in  Alydus  or  Protenor.  Exactly  the 
same  result  is  indicated  in  Archimerus,  where  the  discrepancy  in 
size  between  m-chromosomes  and  heterotropic  chromosome  is 
even  greater  than  in  Anasa  (Fig.  3,  a,  /'). 

3.       BEHAVIOR     OF     THE     HETEROTROPIC     CHROMOSOME     IN     THE 
MATURATION-DIVISIONS    OF    ARCHIMERUS     CALCARATOR. 

In  all  the  Hemiptera  thus  far  described  (Pyrrochoris,  Anasa, 
Alydus,  Protenor,  Syromastes,  Harmostes,  CEdancala,^haries- 
terus),  the  heterotropic  chromosome,  when  present,  divides  equally 
in  the  first  spermatocyte-mitosis,  but  fails  to  divide  in  the  second, 
thus  showing  a  marked  contrast  to  the  phenomena  in  the  Orthop- 
tera  where  the  reverse  order  occurs.  In  the  present  section  I  wish 


Studies  on  Chromosomes. 


525 


briefly  to  record  the  fact  that  Archimerus,  which  agrees  so  closely 
with  Alydus  in  most  other  respects,  differs  from  this  and  all  the 
above-mentioned  forms  in  that  the  heterotropic  chromosome  fails 


k 


d 


/ 


FIGURE  3. 

Archimerus  calcarator. — a,  Side-view  of  first  division  metaphase  showing  heterotropic  chromosome 
and  m-chromosome  bivalent;  b,  polar  view  of  metaphase-group,  first  division;  c,  anaphase  group,  first 
division,  side  view;  d,  late  anaphase,  first  division;  e,  f,  polar  views  of  metaphase-groups,  second 
division,  the  former  including,  the  latter  lacking,  the  heterotropic  chromosome;  g,  spermatocyte- 
nucleus,  prophase  of  first  division,  showing  heterotropic  chromosome  (h\  the  two  separate  m-chromo- 
somes  (m),and  five  of  the  six  large  bivalents;  A, views  of  the  chromosome-nucleolus(heterotropic  chromo- 
some)at  different  periods — i,from  the  contraction-phase  of  the  synaptic  period;  2, middle  growth-period; 
3,  4,  later  growth-period  (the  last  three  showing  central  cavity);  ;',  spermatogonial  metaphase-group. 

to  divide  in  the  first  mitosis,  passing  over  bodily  to  one  pole  and 
dividing  equally  in  the  second  mitosis,  precisely  as  in  the  Orthop- 
tera  (Fig.  3,  c,  </).  This  fact,  which  at  first  I  myself  hardly  found 


526  Edmund  B.  Wilson. 

credible,  is  placed  beyond  doubt  by  numerous  preparations  show- 
ing every  stage  in  the  first  division,  and  no  less  certainly  by  the 
occurrence  of  two  forms  of  the  second  division,  in  equal  numbers 
and  appearing  side  by  side  in  the  same  cyst,  one  of  which  shows 
seven  chromosomes,  the  other  eight,  the  additional  chromosome 
in  the  latter  case  being  usually  recognizable  by  its  size.  Fig.  3, 
c,  d,  shows  two  stages  in  the  history  of  the  heterotropic  chromo- 
some in  the  first  division.  Fig.  3,  e,  /,  gives  polar  views  of  the 
two  forms  of  equatorial  plates  in  the  second  division,  one  showing 
seven,  the  other  eight,  chromosomes.  A  large  number  of  sections 
from  different  individuals  show  no  exception  to  this  mode  of 
distribution,  the  two  divisions  being  immediately  distinguishable 
by  the  size  of  the  cells  and  by  both  the  size  and  the  form  of  the 
chromosomes.  A  similar  case  will  be  described,  in  Banasa 
calva,  in  the  following  section. 

4.      THE    CHROMOSOME-GROUP    IN    BANASA  CALVA. 

In  this  section  I  shall  briefly  describe  a  remarkable  form  that 
is  unique  among  the  Hemiptera  thus  far  described  in  that  it 
possesses  both  the  idiochromosomes  and  a  heterotropic  chro- 
mosome; and  as  a  consequence  of  this  it  is  unique  among  all 
described  animals  in  possessing  not  merely  two  but  four  visibly 
different  classes  of  spermatid-nuclei  in  equal  numbers.  These  four 
classes  are  in  no  visible  way  distinguishable  in  the  fully  formed 
spermatozoa,  but  are  clearly  apparent  in  the  chromosome- 
groups  of  the  spermatid-nuclei. 

No  spermatogonial  metaphase-groups  are  shown  with  sufficient 
clearness  to  admit  of  an  accurate  count,  but  there  are  great 
numbers  of  dividing  spermatocytes  which  show  every  stage  of 
both  the  maturation-divisions.  The  first  division  constantly 
shows,  in  polar  view  of  the  metaphase,  fifteen  chromosomes,  of 
which  two  are  markedly  smaller  than  the  others  (Fig.  4,  #,  £). 
As  is  demonstrated  by  their  later  history,  one  of  these  smaller 
chromosomes  is  the  small  idiochrpmosome  (/)  and  one  the  heter- 
otropic chromosome  (/?).  One  of  them  frequently,  but  not 
invariably,  lies  at  one  side  of  the  group,  sometimes  outside  the 
principal  ring  of  chromosomes  (Fig.  4,  a);  but  it  may  lie  inside 
the  ring  (Fig.  I,  £).  One  always  lies  within  the  ring;  and  judging 
by  the  analogy  of  such  forms  as  Lygaeus,  Euschistus  or  Coenus,  a 


Studies  on  Chromosomes.  527 

much  larger  chromosome  beside  which  it  lies  is  to  be  identified 
as  the  larger  idiochromosome.  Besides  these  fifteen  undoubted 
chromosomes  one  or  more  paler  rounded  bodies  are  often  present, 
lying  outside  the  chromosome-group,  sometimes  close  to  it,  that 
are  undoubtedly  the  remains  of  the  plasmosome  of  the  growth- 
period. 

In  side  views  of  the  metaphase-figure  all  of  these  chromosomes, 
with  one  exception,  have  a  symmetrical  bipartite  (rarely  a  quad- 
ripartite) shape;  and  in  the  ensuing  division  these  are  equally 
divided.  One  of  the  small  chromosomes  (heterotropic)  never 
shows  a  bipartite  shape,  but  is  simply  elongate  and  more  or  less 
fusiform  (Fig.  4,  c,  d,  e).  As  the  division  proceeds,  this  chro- 
mosome at  first  remains  near  the  equator  of  the  spindle  and  then 
passes  over  bodily  toward  one  pole  where  it  enters  the  daughter 
group  (Fig.  4,  /,  £-),  finally  shortening  again  so  as  to  assume 
a  spheroidal  form.  One  of  the  secondary  spermatocytes  there- 
fore receives  fifteen  chromosomes,  the  other  fourteen. 

The  failure  of  this  small  chromosome  to  divide  in  the  first 
mitosis  at  first  seemed  to  me  so  anomalous  (I  had  not  then  observed 
the  similar  phenomenon  in  Archimerus,  described  in  the  foregoing 
section)  that  for  a  time  I  thought  that  this  body  must  be  one  of  the 
fragments  of  the  plasmosome;  and  this  suspicion  was  strengthened 
by  the  fact  that  other  plasmosome-fragments  are  often  found 
lying  near  or  in  the  spindle  (Fig.  4,  g~).  Further  study,  however, 
conclusively  showed  that  this  suspicion  was  not  well-founded. 
The  plasmosome-fragments  are  always  rounded,  paler,  wholly 
inconstant  in  position  and  never  lie  in  the  equatorial  plate.  The 
heterotropic  chromosome,  on  the  other  hand,  is  always  present 
(many  division-figures  in  all  stages  have  been  studied)  and 
every  stage  of  its  asymmetrical  distribution  has  been  repeatedly 
observed.  All  doubt  is,  moreover,  removed  by  a  study  of  the 
metaphase-figures  of  the  second  division.  Great  numbers  of 
these,  showing  the  relations  with  schematic  clearness,  are  avail- 
able for  study.  In  polar  view  these  show  either  fourteen  or 
thirteen  chromosomes  (Fig.  4,  h,  /'),  the  two  classes  existing  in 
approximately  equal  numbers,  and  side  by  side  in  the  same  cyst. 
At  first  sight  neither  of  the  small  chromosomes  of  the  first  division 
can  be  distinguished  in  polar  view  of  the  second.  This  is  owing 
to  two  causes:  First,  the  small  heterotropic  chromosome,  having 
failed  to  divide  while  all  the  others  are  but  half  as  large  as  before, 


528  Edmund  B.  Wilson. 

is  sometimes  hardly  distinguishable  from  the  latter — though,  as  in 
Fig.  4,  /',  it  can  often  be  identified  on  careful  scrutiny.  Second, 
the  small  idiochromosome,  now  only  half  as  large  as  in  the  first 
division,  has  conjugated  in  typical  fashion  with  the  larger  one, 
so  as  to  be  visible,  as  a  rule,  only  in  side  view  (Fig.  4,  /),  though 
careful  focusing  will  often  reveal  it  also  in  polar  view,  especially 
when  the  idiochromosome-dyad  lies  in  a  slightly  oblique  position. 
In  this  way  the  idiochromosome-dyad  has  been  positively  identi- 
fied in  Fig.  4,  h,  /.  In  side-view  the  second  division  shows  with 
entire  clearness  the  separation  of  the  idiochromosome-dyad  into 
its  two  unequal  constituents,  precisely  as  in  Lygaeus,  Euschistus, 
etc.,  while  all  the  other  dyads,  including  the  small  heterotropic, 
divide  equally  (Fig.  4,  /-/).  From  this  it  follows  that  four  visibly 
different  classes  of  spermatid  chromosome-groups  are  formed  in 
equal  numbers.  Two  primary  classes  are  formed  that  possess 
respectively  fourteen  and  thirteen  chromosomes,  according  to  the 
presence  or  absence  of  the  heterotropic  chromosome;  and  each 
of  these  falls  into  two  secondary  classes,  one  of  which  contains 
the  large  idiochromosome,  the  other  the  small.  Although  this 
result  necessarily  follows  from  the  mode  of  division,  it  is  not  a 
matter  merely  of  inference,  but  of  observed  fact;  for  with  a  little 
pains  spindles  of  both  classes  in  the  anaphases  may  readily  be 
found  in  a  vertical  position  that  show  both  the  sister-groups. 
Such  a  pair,  from  the  early  anaphase  of  a  fourteen-chromosome 
spermatocyte,  are  shown  in  Fig.  4,  m,  the  two  groups  exactly 
corresponding,  chromosome  by  chromosome,  except  in  case  of 
the  idiochromosomes  (which  are  shown  by  focusing  to  be  more 
widely  separated  than  the  others).  A  similar  pair  from  a  some- 
what later  anaphase  of  the  thirteen-chromosome  class  is  shown  in 
Fig.  4,  o,  the  relations  being  as  before  save  that  the  heterotropic 
chromosome  is  lacking.  A  pair  from  a  later  anaphase  of  the 
fourteen-chromosome  type  is  shown  in  Fig.  4,  «,  showing  a  prin- 
cipal ring  of  ten  ordinary  chromosomes  within  which  lie  four 
others.  Two  of  these  (below)  are  ordinary  chromosomes;  the 
other  two  are,  at  one  pole  the  heterotropic  and  the  small  idio- 
chromosome, at  the  other  pole  the  heterotropic  and  the  large 
idiochromosome. 


Studies  on  Chromosomes. 


529 


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FIGURE  4. 

Banasa  calva. — a,  b,  Metaphase-figures,  first  division,  in  polar  view,  showing  fifteen  chromosomes, 
including  two  small  ones  (ft,  heterotropic  chromosome,  /',  small  idiochromosome — the  large  idiochro- 
mosome not  distinguishable);  c-g,  successive  stages  of  first  division,  in  side-view,  showing  division  of 
the  small  idiochromosome  («'),  and  the  unipolar  movement  of  the  heterotropic  chromosome  (A);  h,  meta- 
phase-group  of  second  division,  with  thirteen  chromosomes;  «,  metaphase-group  of  the  same  division 
with  fourteen  chromosomes;  _/-/,  metaphase  and  early  anaphase  of  second  division,  showing  separation 
of  the  idiochromosomes,  and  equal  division  of  the  heterotropic  chromosome;  m,  sister-groups  from  the 
same  spindle,  early  anaphase  second  division,  fourteen-chromosome  type;  n,  similar  pair,  late  anaphase; 
o,  similar  pair,  middle  anaphase,  thirteen-chromosome  type;  p,  q,  entire  chromosome-group  from  a 
single  nucleus  at  the  end  of  the  growth-period,  showing  idiochromosome-dyad  (;')  and  heterotropic 
chromosome  (h\ 


53°  Edmund  B.  FFilson. 

The  four  classes  thus  formed  may  be  tabulated  as  follows: 

Primary    Class     A  f  (i)  12  ordinary  chromosomes,  I  heterotropic,  I  large  chromosome. 

(14  chromosomes)  (  (2)  12  ordinary  chromosomes,  I  beterotropic,  I  small  idiochromosome. 
Primary    Class     B  j  (3)  12  ordinary  chromosomes,  I  large  idiochromosome. 

(13  chromosomes)  (^  (4)  12  ordinary  chromosomes,  I  small  idiochromosome. 

Restating  the  facts  from  the  point  of  view  of  mere  size,  it  appears 
that  class  (3)  contains  no  especially  small  chromosome,  class  (2) 
two  small  chromosomes,  and  classes  (i)  and  (4)  each  one  small 
chromosome,  the  latter  being  in  one  case  the  heterotropic,  in  the 
other  the  small,  idiochromosome.1 

I  have  not  yet  studied  in  sufficient  detail  the  history  of  this 
form  in  the  growth-period,  which  will  require  additional  material; 
but  the  main  facts  are  such  as  might  be  expected.  In  the  middle 
growth-period  the  nuclei  show,  with  great  constancy,  two  unequal 
chromosome-nucleoli,  both  of  which  frequently  appear  hollow. 
The  larger  of  these  is  almost  certainly  the  idiochromosome- 
bivalent;  for  in  the  prophases  of  the  first  division  it  may  be  seen 
separating  into  its  two  unequal  constituents,  precisely  as  I 
described  in  Brochymena  (Fig.  4,  /?,  q).  At  this  period  the  hetero- 
tropic chromosome  is  unmistakably  recognizable  by  its  size  and 
shape,  showing  no  constriction  like  that  of  the  other  chromo- 
somes. I  believe  this  to  be  identical  with  the  smaller  chromo- 
some-nucleolus  of  the  earlier  period,  but  cannot  offer  decisive 
proof. 

CRITICAL    AND    COMPARATIVE. 

The  three  well-defined  classes  of  chromosomes  that  have  been 
described  in  this  and  my  preceding  paper  differ  from  the  others, 
each  in  its  own  way,  especially  in  respect  to  their  behavior  in  the 
process  of  synapsis  and  during  the  growth-period.  The  most 
characteristic  common  feature  of  the  first  two  classes  is  their  long 
delayed  synapsis,  which  in  both  cases  is  deferred  to  the  period 

*It  is  probable  that  additional  light  will  be  thrown  on  this  form  by  further  study  of  the  related  one, 
Thyanta  custator,  which  I  now  have  under  investigation.  The  general  aspect  of  the  chromosome  group 
in  this  species  is  closely  similar  to  that  of  Banasa,  and  the  first  mitosis  also  shows  fifteen  chromosomes, 
of  which  however  three,  instead  of  two,  are  smaller  than  the  others.  The  second  mitosis  differs  from 
that  of  Banasa  in  showing  always  but  thirteen  chromosomes,  and  I  have  not  thus  far  found  a  heterotropic 
chromosome  in  either  division.  Though  I  cannot  yet  speak  positively,  these  conditions  seem  only 
explicable  under  the  assumption  that  two  pairs  of  idiochromosomes  are  present.  From  such  a  con- 
dition one  nearly  similar  to  that  observed  in  Banasa  might  readily  be  derived  by  the  disappearance  of 
one  of  the  small  idiochromosomes. 


Studies  on  Chromosomes.  531 

immediately  preceding  the  reduction-division — /.  <?.,  in  case  of  the 
m-chromosomes  to  the  prophases  of  the  first  division,  at  the  very 
end  of  the  growth-period,  and  in  case  of  the  idiochromosomes  to  a 
still  later  period  following  the  first  division  (though  a  temporary 
or  preliminary  union  frequently  occurs  at  a  much  earlier  period). 
The  "accessory"  or  heterotropic  chromosome,  finally,  does  not 
undergo  synapsis  at  all,  since  it  is  without  a  mate  with  which  to 
pair. 

As  regards  their  behavior  in  the  growth-period,  the  idiochro- 
mosomes and  the  heterotropic  chromosome  agree  in  being  "hetero- 
chromosomes"  in  Montgomery's  sense — /.  e.y  are  distinguished 
from  the  other  chromosomes  by  their  compact  form  -and  deep- 
staining  capacity.  The  m-chromosomes,  on  the  other  hand, 
may  remain  in  a  diffused  condition  throughout  the  early  and 
middle  growth-periods,  only  condensing  to  the  compact  form  at 
the  same  time  as  the  ordinary  chromosomes  (Anasa,  "Charies- 
terus");  their  condensation  may,  however,  take  place  in  the 
middle  growth-period  (Alydus),  or  even  earlier  (Protenor,  ac- 
cording to  Montgomery).  An  analogous  difference  in  the  time 
of  condensation  exists  in  case  of  the  idiochromosomes,  which  in 
case  of  Lygaeus  do  not  condense  as  early  as  in  Ccenus  or 
Euschistus. 

My  observations  prove  definitely  in  some  cases  (Alydus, 
Anasa,  Archimerus,  "Chariesterus"),  and  I  think  render  it  prob- 
able for  all  cases,  that  in  those  Hemiptera  that  possess  an  "acces- 
sory" or  heterotropic  chromosome  and  two  equal  spermatogonial 
microchromosomes  (m-chromosomes),  the  large  chromosome- 
nucleolus  of  the  synaptic  and  growth-periods  is  not,  as  other 
observers  have  supposed,  the  microchromosome-bivalent 
("chromatin  nucleolus"  of  Montgomery)  but  the  heterotropic 
chromosome,  precisely  as  in  the  Orthoptera.  This  error  of 
identification  has  led  Montgomery  to  designate  three  quite 
distinct  kinds  of  chromosomes  by  the  same  name  of  "chromatin- 
nucleoli."  These  are  (i)  the  equal  paired  spermatogonial  micro- 
chromosomes  and  the  corresponding  bivalent  of  the  first  sper- 
matocyte  division;  (2)  the  idiochromosomes,  which  are  typically 
unequal  and  do  not  form  a  bivalent  in  the  first  division;  and  (3) 
the  heterotropic  chromosome  as  it  appears  in  the  growth-period. 
It  is  therefore  desirable,  despite  some  repetition,  to  bring  together 
in  brief  form  the  principal  distinctions  between  these  three. 


532  Edmund  B.  Wilson. 

1.  The   paired    microchromosomes — or   preferably   "ra-chro- 
mosomes,"  since  forms  may  be  found  in  which  they  are  not  smaller 
than  the  others — form  an  equal  pair  in  the  spermatogonia,  and  in 
most  of  the  forms  thus  far  known  are  much  smaller  than  the  others. 
These    do    not,   ordinarily  conjugate  to  form  a  bivalent  in  the 
general  synaptic  period,  and  may  (Alydus,  Archimerus)  or  may 
not    (Anasa,    "Chariesterus")    condense    early    in    the    growth- 
period  to  form  two  small  separate  chromosome-nucleoli  which 
can  be  distinguished  in  addition  to  the  principal  one   (hetero- 
tropic  chromosome).     They  undergo  a  very  late  synapsis  (in  the 
prophases  of  the  first  maturation  division)  to  form  a  small  sym- 
metrical bivalent,  typically  central  in  position,  that  undergoes  a 
reduction-division  in  the  first  mitosis  and  an  equation-division 
in  the  second.     Each  spermatid  nucleus  therefore  receives  a  single 
ra-chromosome.     They  are  always,  as  far  as  known,  associated 
with  a  heterotropic  chromosome,  and  the  number  of  spermato- 
gonial  chromosomes  is  odd  (with  the  more  than  doubtful  exception 
of  Syromastes).     The  first  maturation-division  shows  a  number  of 
chromosomes  which  when  doubled  is  one  more  than  the  spermato- 
gonial  number  (as  in  Orthoptera).     Known  to  occur  in  Anasa, 
"Chariesterus,"  Syromastes,  Protenor,  Alydus,  Archimerus,  Har- 
mostes,  (Edancala,  and  doubtless  occur  in  many  others. 

2.  The  idiochromosomes  are  typically  unequal  in  size  (Nezara 
forms  an  exception)  forming  an  unequal  pair  in  the  spermatogonia 
(which  accordingly  show  typically  but  one  small  chromosome); 
they  may  conjugate  to  form  a  bivalent  at  the  time  of  general 
synapsis,  or  may  remain  separate,  in  either  case  condensing  to 
form  a  chromosome-nucleolus   (or  two  separate  unequal  ones) 
which  persists  throughout  the  greater  part  or  the  whole  of  the 
growth-period.     In  either  case  they  are  in  the  Hemiptera  always 
separate  univalents  at  the  time  of  the  first  maturation-mitosis, 
and    separately   undergo    an   equation-division    in   that   mitosis. 
This  division  accordingly  shows  one  more  than  half  the  spermat- 
ogonial    number    of     separate    chromatin-elements,    the    latter 
number  being  in  all  cases  an  even  one.     At  the  end  of  the  first 
mitosis  their  products  conjugate  to  form  a  bivalent  dyad  (thus 
reducing  the  number  of  separate  chromatin-elements  to  one-half 
the  spermatogonial  number).     This  dyad,  typically  unsymmet- 
rical,  undergoes  a  reduction-division  in  the  second  mitosis,  and 
all  of  the  spermatozoa  receive  the  same  number  of  chromosomes, 


Studies  on  Chromosomes.  533 

one-half  receiving  the  larger  and  one-half  the  smaller  idiochro- 
mosome.  They  are  not  ordinarily  associated  with  a  heterotropic 
chromosome,  the  single  known  exception  being  Banasa.  The 
idiochromosomes  are  known  to  occur  in  Lygaeus,  Coenus,  Podisus, 
Trichopepla,  Mineus,  Nezara,  Murgantia,  Brochymena  and 
Banasa  and  are  doubtless  of  much  wider  occurrence. 

3.  The  "accessory"  or  heterotropic  chromosome  is  certainly 
in  most  Hemiptera — and  I  believe  will  be  found  to  be  in  all — 
unpaired  in  the  spermatogonia,  and  its  behavior  is  throughout 
that  of  a  univalent  body.  It  fails  to  unite  in  synapsis  with  any 
other  chromosome,  and  persists  throughout  the  spermatocytic 
growth-period  as  a  chromosome-nucleolus.  During  the  earlier 
part  of  this  period  it  resembles  the  idiochromosome  bivalent  (or 
the  univalent  large  idiochromosome)  in  being  attached  to  a  large 
plasmosome  from  which  it  afterward  separates.1  This  chro- 
mosome divides  in  only  one  of  the  maturation-divisions,  passing 
undivided  to  one  pole  of  the  spindle  in  the  other.  The  latter 
division  is  usually  the  second  (Pyrrochoris,  Anasa,  Protenor, 
Alydus,  Chariesterus,  Syromastes,  Harmostes,  CEdancala),  but 
in  Archimerus  and  Banasa  it  is  the  first.  In  either  case  one-half 
the  spermatozoa  receive  one  more  chromosome  than  the  other 
half. 

From  the  foregoing  it  will  be  seen  that  Montgomery  correctly 
identified  the  chromosome-nucleolus  in  the  growth-period  of 
such  forms  as  Euschistus,  Coenus,  Podisus,  Brochymena,  Tricho- 
pepla or  Nezara,  which  possess  the  idiochromosomes.  He  was, 
however,  at  fault  in  the  conclusion  that  it  gave  rise  to  a  small 
bivalent  in  the  first  division,  the  small  chromosome  of  this  division 
being  always  a  univalent  that  is  not  at  this  time  paired  with  its 
(usually)  larger  fellow;  and  further,  owing  to  a  failure  to  discrimi- 
nate between  these  bodies  and  the  paired  microchromosomes  of 
the  Anasa  or  Alydus  type,  he  describes  and  figures  the  spermat- 
ogonial.  groups  in  most  of  these  forms  as  containing  a  symmetrical 
pair  of  "chromatin-nucleoli."  Owing  to  his  having  overlooked 
the  constant  separateness  of  the  idiochromosomes  as  univalents 
in  the  first  mitosis  he  has  also,  I  believe,  been  misled  in  several 


'It  is  doubtless  a  similar  condition  that  has  led  Moore  and  Robinson  ('05)  in  the  case  of  Periplaneta, 
to  conclude  that  the  "accessory''  chromosome  is  nothing  but  a  "nucleolus."  These  observers  have 
evidently  studied  the  phenomena  in  a  very  superficial  manner. 


534  Edmund  B.  Wilson. 

instances  in  regard  to  the  spermatogonial  number  (e.  g.,  in 
Euschistus  variolarius,  Nezara  and  Brochymena).  The  state- 
ment given  in  the  general  summing  up  of  his  latest  paper  ('05) 
"Whenever  the  heterochromosomes  occur  in  pairs  in  the  sper- 
matogonia  they  (/.  e.,  the  'chromatin  nucleoli')  always  conjugate 
to  form  bivalent  ones  in  the  first  spermatocytes,  and  their  univalent 
components  become  separated  in  the  first  maturation  mitosis,  /.  e., 
divide  prereductionally"  (p.  195,  and  elsewhere),  is  inapplicable 
to  the  idiochromosomes;  for  even  though  they  conjugate  to  form 
a  bivalent  chromosome-nucleolus  in  the  growth-period  they  again, 
separate  to  divide  as  separate  univalents  in  the  first  mitosis,  as  I 
showed  in  detail  in  Brochymena,  and  as  must  also  occur  in  the 
other  forms  (as  is  proved  by  the  number  of  the  chromosomes  and 
their  later  history).  The  statement  cited  above  applies  only  to 
the  w-chromosomes  of  such  forms  as  Anasa,  Chariesterus,  Alydus, 
Archimerus  or  Protenor;  but  the  name  "chromatin  nucleoli"  is 
in  these  cases  not  very  appropriate  in  view  of  the  fact  that  in  the 
very  form  (Anasa)  in  which  they  were  first  discovered  they  do  not 
appear  as  chromatin-nucleoli  at  any  time  during  the  growth- 
period  of  the  spermatocytes.  As  to  their  behavior  in  the  rest- 
period  of  the  spermatogonia  I  have  at  present  no  opinion  to-express. 
It  is  further  probable  that  the  distinction  urged  by  Montgomery 
between  the  "odd  chromosome"  and  the  accessory  ('05,  p.  192) 
is  also  not  valid;  for  my  observations  prove  that  in  Alydus  and 
Archimerus  the  "odd  chromosome"  ("accessory")  is  a  typical 
chromosome-nucleolus  (i.  e.,  "heterochromosome")  in  the  growth- 
period,  and  it  is  extremely  probable  that  the  same  will  be 
found  to  hold  true  of  the  "odd  chromosome"  of  Harmostes  and 
QEdancala.  I  think  therefore  that  Montgomery's  general  con- 
clusions regarding  the  "heterochromosomes"  require  some 
revision. 

We  may  now  briefly  consider  the  nature  of  the  "accessory"  or 
heterotropic  chromosome.  So  long  as  any  of  the  forms  possessing 
such  a  chromosome  were  supposed  to  have  an  even  number 
of  spermatogonial  chromosomes  the  conclusion  drawn  by  Mont- 
gomery ('01,  '04,  '05)  that  this  chromosome  is  a  bivalent  seemed 
an  almost  necessary  one,  even  in  cases  where  it  appears  as  a 
single  body  in  the  spermatogonia.  The  observations  brought 
forward  in  this  paper  cast  grave  doubt,  I  think,  on  all  of  the 
earlier  accounts  asserting  an  even  spermatogonial  number  in 


Studies  on  Chromosomes.  535 

the  Hemiptera  that  possess  a  heterotropic  chromosome.  Of 
these  accounts  (in  cases  positively  known  to  have  such  a  chro- 
mosome) there  are  but  four,  namely,  Henking's  original  account 
of  Pyrrochoris  ('90),  Paulmier's  ('99),  and  Montgomery's  ('01, 
'04)  accounts  of  Anasa,  Montgomery's  of  Alydus  pilosulus  ('01) 
and  Gross's  more  recent  one  of  Syromastes  ('04).  Henking  states 
that  he  counted  but  four  cases,  one  of  which  seemed  to  show 
twenty-three,  the  other  three  twenty-four,  and  it  is  evident  both 
from  the  figures  and  from  the  frank  statement  of  this  able  observer, 
that  he  adopted  the  latter  number  more  on  account  of  theoretical 
considerations  than  as  a  result  of  any  adequate  study  of  the  facts. 
I  have  shown  the  counts  of  Paulmier  and  Montgomery  to  be 
erroneous  in  the  case  of  Anasa,  and  also  that  of  Montgomery  in 
the  case  of  Alydus  pilosulus.  There  remains  therefore  the  single 
case  of  Syromastes;  but  perhaps,  in  view  of  the  results 
I  have  reached  in  other  forms,  I  may  be  allowed  the  pre- 
diction that  a  reexamination  of  this  one  will  lead  to  a  similar 
conclusion. 

If  this  expectation  is  verified  every  ground  will  be  removed  for 
considering  the  heterotropic  chromosome  as  a  bivalent  body; 
and  I  think  that  until  definite  evidence  to  the  contrary  is  forth- 
coming we  are  bound  to  take  this  chromosome  at  its  face-value, 
so  to  speak,  as  univalent.  This  conclusion  involves  a  series  of 
other  conclusions  and  possibilities  of  which  I  shall  here  undertake 
to  indicate  only  the  more  important. 

1.  As  was  indicated  by  McClung  ('02,  p.  71),  if  the  "accessory" 
be  univalent,  its  behavior  in  the  maturation-mitoses  at  once  falls 
into  line  with  that  of  the  other  spermatogonial  chromosomes;  for 
each  of  these,  too,  undergoes  but  one  division  in  the  course  of  the 
two  maturation-mitoses.     One  of  these  divisions  (the  reduction 
division)  merely  separates  the  univalent  chromosomes  that  have 
previously  paired  in  synapsis  (as  is  so  convincingly  shown  in  case 
of  the  idiochromosomes  or  the  m-chromosomes);    and  only  the 
fact  that  the  "accessory"  has  no  mate  with  which  to  pair  renders 
its  behavior  in  one  of  the  divisions  apparently  different  from  that 
of  the  ones  that  do  pair. 

2.  The  objections  that  I  myself  urged  to  the  suggestion  made 
in  the  first  of  these  studies  regarding  the  origin  of  the  heterotropic 
chromosome  are  thus  set  aside,  and  my  attempt  to  compare  the 
idiochromosomes  with  the  m-chromosomes  was  made  on  incorrect 


536  Edmund  B.  Wilson. 

premises.  My  suggestion  was  that  a  heterotropic  chromosome 
might  arise  from  a  symmetrical  bivalent  by  the  gradual  reduction 
and  final  disappearance  of  one  member  of  the  conjugating  pair, 
conditions  corresponding  to  several  of  the  stages  of  such  a  reduc- 
tion being  shown  to  exist  in  Nezara,  Mineus,  Ccenus,  Euschistus, 
Murgantia,  and  Lygaeus.  All  of  the  facts  seem  to  me  to  indicate 
that  this  interpretation  is  the  true  one.  Were  the  small  idio- 
chromosome  to  disappear  in  such  forms  as  Lygaeus  or  Euschistus, 
the  large  idiochromosome  would  be  left  as  a  heterotropic  chro- 
mosome agreeing,  point  by  point,  with  that  of  such  forms  as 
Alydus,  Protenor  or  Anasa,  namely,  in  its  persistence  as  a  chro- 
mosome-nucleolus  during  the  growth-period;  its  association  with 
the  plasmosome  in  the  earlier  part  of  this  period  and  its  subse- 
quent separation  from  it;  its  equal  bipartition  by  an  equation- 
division  in  the  first  spermatocyte-mitosis,  and  the  failure  of  the 
resulting  products  to  divide  in  the  second  mitosis;  and  in  corre- 
lation with  the  foregoing  the  existence  of  an  odd  number  of 
spermatogonial  chromosomes.  The  exactness  of  this  corre- 
spondence is  such,  I  think,  as  to  lend  a  high  degree  of  probability 
to  the  interpretation. 

The  only  apparent  obstacle  in  its  way  is  the  fact  that  in  Banasa 
a  heterotropic  chromosome  coexists  with  a  typical  pair  of  idio- 
chromosomes;  but  this  difficulty  only  exists  under  the  assumption 
that  a  heterotropic  chromosome  has  arisen  but  once  in  the  history 
of  the  species,  and  nothing  is  known  to  justify  such  an  assumption. 
I  think,  on  the  contrary,  that  the  facts  in  Banasa  may  fairly  be 
taken  as  evidence  that  a  process  is  here  in  progress  which  if  con- 
tinued would  lead  to  the  formation  of  a  second  heterotropic 
chromosome.1 

3.  The  formation  of  a  heterotropic  chromosome  in  the  manner 
indicated  involves  a  reduction  of  the  total  number  of  chromo- 
somes by  one;  and  it  is  possible  that  this  may  represent  one  process 
by  which  changes  from  a  higher  to  a  lower  number  or  chromo- 
somes have  been  brought  about.  But  I  doubt  whether  such  a 
process  can  have  gone  very  far,  since,  as  pointed  out  beyond, 
there  is  reason  to  believe  that  it  has  occurred  in  only  one  sex. 


Should  my  surmise  (stated  in  the  footnote  at  p.  530)  be  correct  that  in  the  related  form  Thyanta 
two  pairs  of  idiochromosomes  are  present  without  a  heterotropic  chromosome,  I  think  additional  support 
will  be  lent  to  the  above  interpretation. 


Studies  on  Chromosomes.  537 

It  seems,  on  the  other  hand,  probable  that  the  m-chromosomes 
may  be  of  more  general  significance  in  this  direction,  since  the 
facts  distinctly  suggest  that  they  are  diminishing  or  disappearing, 
and  perhaps  in  some  cases  already  vestigial,  structures  in  both 
sexes.  Paulmier  was  the  first,  as  far  as  I  am  aware,  to  suggest 
that  a  reduction  in  the  size  of  particular  chromosomes  might  fore- 
shadow their  total  disappearance;  that  chromosomes  might  in 
this  way  assume  a  vestigial  character;  and  further,  that  such 
chromosomes  might  represent  "somatic  characters  which  belonged 
to  the  species  in  former  times,  but  which  characters  are  disappear- 
ing" ('99,  p.  261).  This  conception  was  applied  by  him  to  the 
small  ra-chromosomes  (which  he  believed  to  represent  the  "acces- 
sory"), but  was  further  supported  by  his  observation  of  a  very 
small  chromatin-body  that  may  divide  like  a  chromosome  (Paul- 
mier, Fig.  28,  a)  but  is  only  rarely  visible.1  Paulmier's  suggestion, 
which  I  suspect  may  prove  to  embody  one  of  the  most  important 
results  of  his  paper,  has  been  further  developed  by  Montgomery. 
This  author  first  suggested  that  an  uneven  number  of  chro- 
mosomes "represents  a  transition  stage  between  a  higher  number 
and  a  lower"  ('01,  p.  215);  and  he  has  more  recently  assumed 
that  the  "unpaired  heterochromosomes"  ("accessory"  or  hetero- 
tropic  chromosomes)  have  arisen  from  paired  heterochromosomes 
("chromatin  nucleoli")  or  ordinary  chromosomes  by  fusion  of 
the  members  of  a  pair  to  form  a  bivalent  body  ('05,  p.  197). 
Both  the  paired  and  the  unpaired  heterochromosomes  are  con- 
sidered to  be  chromosomes  on  the  way  to  disappearance.  Though 
my  conclusion  regarding  the  origin  of  the  unpaired  or  heterotropic 
chromosome  is  an  entirely  different  one,  it  agrees  with  that  of 
Montgomery  in  assuming  a  reduction  in  the  original  number  of 
chromosomes;  and  it  is  possible  that  by  a  subsequent  disappear- 
ance of  the  heterotropic  chromosome  a  further  reduction  may  take 
place,  though  as  indicated  above  there  are  difficulties  in  the  way 
of  this  assumption.  My  conclusion  is,  however,  distinctly  opposed 
to  the  view  that  heterotropic  chromosomes  have  arisen  from 
"paired  heterochromosomes"  (ra-chromosomes),  and  although 
they  have  some  features  in  common  the  evidence  is  opposed  to 


xlt  seems  quite  possible  that  this  body  may  be  the  last  remnant  of  a  small  idiochromosome,  of  which 
the  corresponding  larger  one  has  remained  as  the  heterotropic  chromosome;  but  definite  evidence  of 
this  is  lacking. 


538  Edmund  B.  Wilson. 

any  direct  relationship  between  these  two  classes  of  chromosomes. 
Montgomery  has  called  attention  to  the  fact  that  the  ra-chro- 
mosomes  vary  greatly  in  size  in  different  species,  graduating  down 
to  excessively  minute  forms  (such  as  those  occurring  in  Archi- 
merus.)  It  is  evident  that  these  chromosomes  have  undergone 
a  symmetrical  reduction  which,  if  continued,  might  lead  to  the 
disappearance  of  both;  and  such  a  process,  if  repeated,  would 
lead  in  the  history  of  a  species  to  a  progressive  and  parallel 
reduction  of  the  number  in  both  sexes.  When  these  facts  are 
compared  with  those  presented  by  the  idiochromosomes  the 
thought  can  hardly  be  avoided  that  the  reduction  of  the  m-chro- 
mosomes  may  be  correlated  with  a  corresponding  change  that  is 
taking  place  equally  in  both  sexes;  while  the  reduction  of  the 
small  idiochromosome  may  represent  a  change  that  is  taking  place 
more  rapidly  in  one  sex  than  in  the  other,  or  affects  one  sex  only. 
4.  How  the  foregoing  conclusions  and  suggestions  regarding 
the  idiochromosomes  and  heterotropic  chromosomes  will  square 
with  McClung's  hypothesis  ('02,  2)  and  my  own  similar  sug- 
gestion ('05)  that  these  bodies  may  be  in  some  way  concerned 
with  sex-determination,  does  not  yet  clearly  appear  from  the 
known  data;  but  there  are  some  considerations  that  are  too 
interesting  in  this  connection  to  be  ignored.  If  the  heterotropic 
chromosome-  be  a  univalent  body  the  conclusion  is  unavoidable 
(since  the  spermatogonial  number  is  odd)  that  in  the  production 
of  males,  the  number  of  chromosomes  contributed  by  the  two 
germ-cells  cannot  be  the  same.  To  this  extent  the  facts -har- 
monize with  the  view  of  McClung;  but  further  consideration 
gives  reason  to  doubt  some  of  the  more  specific  features  of  his 
hypothesis.  The  presence  of  the  heterotropic  chromosome  in 
the  male  by  no  means  proves  that  it  is  of  paternal  origin  in  fer- 
tilization, still  less  that  it  is  specifically  the  male  sex-determinant— 
indeed,  I  believe  the  facts  point  in  the  opposite  direction.  In 
Anasa,  for  example,  where  the  spermatozoa  possess  either  ten 
or  eleven  chromosomes,  offspring  (males)  having  twenty-one 
would  be  produced  by  the  fertilization  of  an  egg  having  ten  chro- 
mosomes by  a  spermatozoon  having  eleven  (as  McClung  would 
assume);  but  the  same  result  would  follow  from  the  fertilization 
of  an  egg  having  eleven  by  a  spermatozoon  having  ten.  I  believe 
the  second  of  these  alternatives  to  be  the  more  probable  one  for 
the  following  reasons:  According  to  my  view,  the  heterotropic 


Studies  on  Chromosomes.  539 

chromosome  has  assumed  its  unpaired  character  by  the  reduction 
and  final  disappearance  of  its  parental  mate  or  homologue  (z.  e.,  a 
small  idiochromosome);  and  it  is  highly  probable  that  this  pro- 
cess has  occurred  in  one  sex  only,  namely,  the  male.1  If  this  be 
the  fact,  it  is  evident  that  the  heterotropic  chromosome  that 
remains  in  the  male  is  the  maternal  mate  or  homologue  of  that 
which  has  vanished.  I  think  therefore  that  we  may  expect  to  find 
that  the  heterotropic  chromosome  present  in  the  male  is  derived 
in  fertilization  from  the  maternal  group  of  chromosomes;  and 
also  that  the  female  will  be  found  to  possess  one  more  chromosome 
than  the  male  (exactly  the  opposite  of  McClung's  assumption), 
the  additional  chromosome  being  the  homologue  of  that  which 
has  vanished  in  the  male.2  If  this  be  the  fact,  it  follows  with 
great  probability  that  in  the  egg-synapsis  this  chromosome  pairs 
with  its  paternal  homologue  (originally  the  heterotropic  chro- 
mosome) to  form  a  symmetrical  bivalent,  and  that  all  the  eggs 
receive  eleven  chromosomes;  while  in  the  male  the  heterotropic 
chromosome  fails  to  pair  (having  no  mate)  and  hence  remains 
univalent.  The  expectation  may  therefore  be  stated  as  follows: 

Egg  ii  +  spermatozoon  10  =•  21  (male). 
Egg  ii  +  spermatozoon  n  =  22  (female).8 

Important  direct  evidence  in  favor  of  this  expectation  is  given 
by  the  discovery  by  Stevens,  briefly  referred  to  in  my  preced- 
ing paper,  that  in  the  beetle  Tenebrio  a  small  chromosome, 
evidently  analogous  to  the  small  idiochromosome  of  Hemiptera, 
is  present  in  the  somatic  cells  of  the  male  only,  while  in  the  female 

!I  will  here  not  go  into  the  somewhat  intricate  difficulties  encountered  under  the  supposition  that 
it  has  occurred  in  both  sexes,  except  to  point  out  that  if  an  unpaired  heterotropic  chromosome  be  present 
in  the  female  and  is  allotted  to  only  half  the  eggs  (as  in  the  male)  it  is  necessary  to  assume  a  fertiliza- 
tion of  each  form  of  egg  by  the  opposite  form  of  spermatozoon,  since  otherwise  three  forms  of  offspring 
would  result.  Such  a  mode  of  fertilization  is  a  priori  very  improbable.  Still  greater  difficulties  stand 
in  the  way  of  assuming  that  an  unpaired  heterotropic  chromosome,  present  in  the  female,  is  retained  in 
all  of  the  eggs. 

2Montgomery  ('04)  has  in  fact  found  in  the  oogonia  and  follicle-cells  of  the  female  Anasa  twenty- 
two  chromosomes,  and  Gross  ('04)  reports  the  same  number  in  those  of  the  female  Syromastes.  But 
since  the  first-named  observer  is  certainly,  and  I  believe  the  second-named  is  probably,  in  error  as  to 
the  number  in  the  male,  both  these  cases  require  reexamination.  On  the  other  hand  Sutton  has  found 
twenty-two  in  the  oogonia  and  follicle-cells  of  the  Orthoptera  (Brachystola)  while  the  spermatogonial 
groups  show  twenty-three;  but  here  again  I  think  a  result  so  important  should  be  supported  by  more 
adequate  evidence  than  he  has  brought  forward.  I  now  have  this  subject  under  investigation. 

"For  the  confirmation  of  this,  see  Appendix. 


54°  Edmund  B.  Wilson. 

it  is  represented  by  a  corresponding  larger  one  (both  sexes  having 
the  same  number  of  chromosomes).  Were  the  small  chromo- 
some to  disappear,  the  female  would  show  one  more  chromosome 
than  the  male  in  accordance  with  my  general  assumption. 

We  have  now  therefore  good  reason  to  hope  that  observation 
will  directly  determine  whether  sex  is  predetermined  in  the  chro- 
mosome-group; and  further,  whether  the  sex-determining  func- 
tion can  be  localized  in  a  particular  chromosome  or  pair  of 
chromosomes,  as  McClung  suggested. 

5.  The  foregoing  offers  no  specific  suggestion  as  to  the  mean- 
ing of  the  four  classes  of  spermatozoa  observed  in  Banasa.  But  it 
may  be  remarked  that  the  existence  of  two  or  four  (or  more) 
classes  of  germ-cells  in  the  same  sex  is  in  itself  nothing  anomalous; 
for  as  Sutton  has  pointed  out,  under  the  conception  of  himself 
and  Montgomery  there  may  be  as  many  classes  of  spermatozoa 
as  there  are  combinations  of  paternal  and  maternal  chromo- 
somes (in  accordance  with  the  Mendelian  ratios).  Forms  which 
possess  idiochromosomes  or  heterotropic  chromosomes  differ 
from  the  more  usual  ones  only  in  that  two  or  four  of  these  classes 
are  made  visible  by  a  greater  or  less  differentiation  of  the  members 
of  one  or  two  of  the  chromosome-pairs.  It  seems  admissible  to 
suppose  that  such  a  visible  differentiation  of  the  members  of 
particular  chromosome-pairs  may  stand  for  a  corresponding 
differentiation  of  corresponding  or  allelomorphic  qualities  in  the 
adult.  I  would  therefore  suggest  the  possibility  that  such  a 
visible  polymorphism  of  the  male  germ-nuclei  as  exists  in  Banasa 
may  be  accompanied  by  a  visible  polymorphism  in  the  adults; 
and,  while  I  am  not  aware  that  such  a  polymorphism  has  been 
observed  in  the  Hemiptera,  I  believe  this  subject  should  be  care- 
fully examined. 

It  is  hardly  necessary  to  point  out,  finally,  how  strong  a  support 
the  foregoing  observations  lend  to.  the  general  hypothesis  of  the 
individuality  of  chromosomes,  and  to  the  conception  of  synapsis 
and  reduction  first  brought  forward  by  Montgomery  and  developed 
in  so  fruitful  a  way  by  Sutton  and  Boveri.  I  must  frankly  confess 
that  until  I  had  followed  step  by  step  the  behavior  of  the  idiochro- 
mosomes and  the  ra-chromosomes  in  the  Hemiptera  I  did  not  appre- 
ciate how  cogent  is  the  argument  brought  forward  in  Montgomery's 
paper  of  '01  in  support  of  his  conclusion  that  synapsis  involves  an 
actual  conjugation  of  chromosomes  two  by  two,  and  that  the 


Studies  on  Chromosomes.  541 

chromosomes  thus  uniting  are  the  paternal  and  maternal  homo- 
logues.  In  the  case  of  the  m-chromosomes,  no  less  clearly  than 
in  that  of  the  idiochromosomes,  the  conjugation  is  not  in  anyway 
an  inference  but  an  easily  observed  fact;  and  in  both  cases  it  is 
equally  clear  that  the  subsequent  reducing  division  separates, 
with  their  individuality  unimpaired,  the  same  chromosomes  that 
have  previously  united  in  synapsis. 

I  believe  that  any  observer  who  will  take  the  trouble  to  study 
in  detail  the  history  of  the  chromosomes  in  these  insects  must  sooner 
or  later  in  his  task  acquire  the  firm  conviction  that  he  is  dealing 
with  definite,  well  characterized,  entities  which  show  the  most 
marked  individual  characteristics  of  behavior,  which  in  some 
manner  persist  from  one  cell-generation  to  another  without  loss 
of  their  specific  character,  and  which  unite  in  synapsis  and  are 
distributed  in  the  ensuing  maturation-divisions  in  a  perfectly 
definite  manner.  All  the  facts  indicate  that  these  phenomena  are 
the  visible  expression  of  a  preliminary  association,  and  subsequent 
distribution  to  the  germ-cells,  of  corresponding  hereditary  char- 
acters. It  is  evident,  therefore,  that  the  time  has  come  when 
cytologists  must  seriously  set  themselves  to  the  task  of  working 
out  a  comparative  morphology  and  physiology  of  the  chromosomes, 
with  the  ultimate  aim  of  attempting  their  specific  correlation  with 
the  phenomena  of  heredity  and  development. 

SUMMARY. 

1.  The  chromosomes  that  have   been   called  "  heterochromo- 
somes"  in  Hemiptera  (Montgomery)  include  three  distinct  forms 
that  may  provisionally  be  called  (a)  the  paired  microchromosomes 
or  m-chromosomes;  (&)  the  idiochromosomes;   (c)  the  "accessory" 
or  heterotropic  chromosomes. 

2.  The  m-chromosomes  are  usually  very  small,  form  a  sym- 
metrical  pair  in  the   spermatogonia,   and   do   not   unite  (in   the 
forms  I  have  studied)  to  form  a  bivalent  chromosome-nucleolus 
in  the  growth-period.     At  an  earlier  or  later  period  they  condense 
to  form  two  separate  chromosomes  that  finally  pair  to  form  the 
small  bivalent  central  of  the  first  division,  but  are   immediately 
separated  without  fusion.     Each   divides   equally  in  the  second 
division. 

3.  The   idiochromosomes   are  typically  unequal,   and   hence 
do  not  form  a  symmetrical  pair  in  the  spermatogonia.     They  may 


542  Edmund  B.  Wilson. 

or  may  not  pair  at  the  time  of  general  synapsis  to  form  a  bivalent; 
in  the  former  case  they  appear  in  the  growth-period  as  a  single 
bivalent  chromosome-nucleolus,  in  the  latter  case  as  two  separate 
univalent  chromosome-nucleoli.  In  either  case  they  undergo 
equal  division  as  separate  univalents  in  the  first  maturation- 
mitosis,  their  products  conjugating  at  the  close  of  this  division  to 
form  an  asymmetrical  dyad  the  two  constituents  of  which  are, 
without  fusion,  immediately  separated  in  the  second  division. 

4.  The  heterotropic  chromosome  is  without  a  mate  in  the 
spermatogonia  (which  accordingly  show  an  odd  number  of  chro- 
mosomes) and  hence  fails  to  undergo  synapsis.     Its  behavior  is 
throughout  that  of  a  univalent  body.     It  divides  only  once  in  the 
course  of  the  two  maturation  mitoses,  this  division  taking  place 
usually  in  the  first,  but  in  some  species  in  the  second,  mitosis. 
It  has  probably  arisen  by  the  reduction  and  final  disappearance 
of  one  member  of  a  symmetrical  chromosome-pair,  this  process 
having  taken  place  in  the  male  only. 

5.  The  w-chromosomes  are  always  associated  with  a  hetero- 
tropic chromosome,  while  the  idiochromosomes  and  heterotropic 
chromosomes  are  known  to  coexist  in  only  a  single  case  (Banasa). 
This  case  indicates  that  the  formation  of  heterotropic  chromo- 
somes may  have  taken  place  more  than  once  in  the  history  of  the 
species  and  possibly  represents  one  mode  of  change  from  a  higher 
to  a  lower  number  of  chromosomes. 

6.  In  forms  possessing  the  idiochromosomes  two  classes  of 
spermatozoa   exist   in   equal   numbers,  which   receive   the  same 
number  of  chromosomes  but  differ  in  respect  to  the  idiochro- 
mosome.     In  forms  possessing  a  heterotropic  chromosome  two 
classes  of  spermatozoa  likewise  exist,  one  of  which  possesses  one 
more  chromosome  than  the  other.     When  both  idiochromosomes 
and  heterotropic  chromosomes  are  present  (Banasa)  four  classes 
of  spermatozoa  are  formed,  two  having  one  more  chromosome  than 
the  other  two,  each  of  these  groups  again  differing  in  respect  to 
the  idiochromosome. 

7.  The  facts  support  the  general  theory  of  the  individuality 
of  chromosomes,  the  theory  of  Montgomery  in  regard  to  synapsis, 
and  that  of  Sutton  and  Boveri  regarding  its  application  to  Men- 
delian  inheritance;  and  they  point  toward  a  definite  connection 
between  the  chromosome-group  and  the  determination  of  sex. 

Zoological  Laboratory,  Columbia  University, 
July  29th,  1905. 


Studies  on  Chromosomes.  543 


APPENDIX. 


During  the  summer,  and  since  the  foregoing  paper  was  entirely 
completed  in  its  present  form,  I  have  obtained  new  material  which 
shows  decisively  that  the  theoretic  expectation  in  regard  to  the 
relations  of  the  nuclei  in  the  two  sexes,  stated  at  p.  539,  is 
realized  in  the  facts.  In  Anasa,  precisely  in  accordance  with  the 
expectation,  the  oogonial  divisions  show  with  great  clearness  one 
more  chromosome  than  the  spermatogonial,  namely,  twenty-two  in- 
stead of  twenty-one;  and  the  same  number  occurs  in  the  divisions 
of  the  ovarian  follicle-cells.  Again  in  accordance  with  the  expec- 
tation, the  oogonial  groups  show  four  large  chromosomes  instead 
of  the  three  that  are  present  in  the  spermatogonial  groups.  In 
other  respects  the  male  and  female  groups  are  closely  similar.  In 
like  manner,  the  oogonial  divisions  in  Alydus  and  Protenor  show 
fourteen  chromosomes,  the  spermatogonial  but  thirteen;  and  in 
Protenor  the  spermatogonial  chromosome-groups  have  but  one 
large  chromosome  (unquestionably  the  heterotropic)  while  the 
oogonial  groups  have  two  such  chromosomes  of  equal  size. 

The  interpretation  is  unmistakable.  Taking  Protenor  as  a 
type,  all  of  the  matured  eggs  must  contain  seven  chromosomes, 
of  which  one,  much  larger  than  the  others,  corresponds  to  the 
heterotropic  chromosome  present  in  one-half  of  the  spermatozoa. 
These  spermatozoa  (seven-chromosome  forms)  contain  a  chromo- 
some-group exactly  similar  to  that  of  the  egg;  and  fertilization  by 
a  spermatozoon  of  this  class  produces  a  female  having  fourteen 
chromosomes.  The  other  half  of  the  spermatozoa  (six-chromo- 
some forms)  lack  the  heterotropic  chromosome;  and  fertilization 
of  an  egg  by  a  spermatozoon  of  this  class  produces  a  male  having 
but  thirteen  chromosomes,  the  unpaired  one  being  derived  from 
the  egg  and  appearing  in  the  maturation  of  this  male  as  the 
heterotropic  chromosome  since  it  is  without  a  mate.  There  can, 
therefore,  be  no  doubt  that  a  definite  connection  exists  between 
the  chromosomes  and  the  sexual  characters,  and  I  believe  that 
the  conclusion  can  hardly  be  escaped  that  the  chromosome- 
combination,  established  at  the  time  of  fertilization,  is,  in  these 
insects,  the  determining  cause  of  sex. 

The  result  reached  in  Anasa  is  confirmed  by  a  comparison  of 
the  male  and  female  chromosome-groups  in  Lygaeus,  Ccenus  and 
Euschistus,  all  of  which  possess  in  the  male  a  pair  of  unequal 


544  Edmund  B.  Wilson. 

idiochromosomes  in  place  of  an  unpaired  heterotropic  chromosome. 
In  all  of  these  forms,  as  I  showed  in  my  first  paper,  the  spermato- 
gonial  groups  show  fourteen  chromosomes  that  may  be  equally 
paired  with  the  exception  of  a  small  and  a  large  idiochromosome. 
The  oogonial  groups  in  these  forms  also  show  fourteen  chromo- 
somes, but  all  may  be  equally  paired,  the  small  idiochromosome 
being  represented  by  a  larger  one  that  has  a  mate  of  equal  size. 
In  these  forms,  accordingly,  males  are  produced  as  a  result  of 
fertilization  by  spermatozoa  containing  the  small  idiochromosome, 
females  by  fertilization  by  spermatozoa  containing  the  large  idio- 
chromosome (which  accords  with  Stevens'  result  in  Tenebrio). 
This  proves  the  correctness  of  my  conclusion  that  the  size-reduction 
and  final  disappearance  of  the  small  idiochromosome  has  taken 
place  in  the  male  sex  only,  and  that  the  large  idiochromosome 
corresponds  to  the  heterotropic  chromosome.  Complete  disap- 
pearance of  the  small  idiochromosome  in  the  male  has  led  to 
each  a  condition  as  exists  in  Anasa  and  other  forms  possessing  a 
heterotropic  chromosome.  These  facts  will  be  described  and 
discussed  in  the  third  of  these  studies. 

October  4,  1905. 


Studies  on  Chromosomes.  545 

LITERATURE-1 

BAUMGARTNER,  W.  J.,  '04. — Some  new  Evidences  for  the  Individuality  of  the 

Chromosomes.     Biol.  Bull.,  viii,  I. 
BOVERI,  TH.,  '04. — Ergebnisse  iiber  die  Konstitution  der  Chromatischen  Substanz 

des  Zellkerns.     Jena,  1904. 
GROSS,  J.,  '04. — Die  Spermatogenese  von  Syromastes  marginatus.     Zool.  Jahrb., 

Anat.  Ontog.,  xx,  3. 
HENKING,  H.,  '90. — Ueber  Spermatogenese  und  deren  Beziehungzur  Entwickelung 

bei  Pyrrochoris  apterus.     Z.  wiss.  Zool.,  li. 
McCLUNG,  C.  E.,  'oo. — The  Spermatocyte  Divisions  of  the  Acrididae.     Bull.  Univ. 

Kansas,  ix,  I. 

'02,  I. — The  Spermatocyte  Divisions  of  the  Locustidae.     Ibid.,  xi,  8. 
'02,  2. — The  Accessory  Chromosome.     Sex  Determinant?     Biol.  Bull., 

iii,  i,  2. 
MONTGOMERY,  T.  H.,  '98. — The  Spermatogenesis  in  Pentatoma,  etc.     Zool.  Jahrb., 

Anat.  Ontog.,  xii. 
'01. — A  Study  of  the  Chromosomes  of  the  Germ-cells  of  Metazoa.     Trans. 

Amer.  Phil.  Soc.,  xx. 
'04. — Some    Observations    and    Considerations    upon    the    Maturation 

Phenomena  of  the  Germ-cells.     Biol.  Bull.,  vi,  3. 
'05. — The  Spermatogenesis  of  Syrbula  and  Lycosa,  etc.     Proc.  Acad. 

Nat.  Sci.  Phil.,  Feb.,  1905.     Issued  May  18,  1905. 

MOORE  AND  ROBINSON,  '05. — On  the  Behavior  of  the  Nucleolus  in  the  Spermato- 
genesis of  Periplaneta  Americana.     Q.  J.  M.  S.,  xlviii,  4. 
PAULMIER,  F.  C.,  '99. — The  Spermatogenesis  of  Anasa  tristis.     Jour.  Morph., 

xv,  supplement. 
STEVENS.,  N.  M.,  '05. — A  Study  of  the  Germ-cells  of  Aphis  rosae  and  Aphis  ceno- 

therae.     Journ.  Exp.  Zool,  ii,  3 
SUTTON,  W.  S.,  'oo. — The  Spermatogonial  Divisions  in  Brachystola  magna.     Bull. 

Univ.  Kansas,  ix,  I. 
'02. — On  the  Morphology  of  the  Chromosome  Group  in  Brachystola 

magna.     Biol.  Bull.,  iv,  i. 

'03. — The  Chromosomes  in  Heredity.     Biol.  Bull.,  iv,  5. 

WILSON,  E.  B.,  '05. — The  Behavior  of  the  Idiochromosomes  in  Hemiptera.    Journ. 
Exp.  Zool.,  ii,  3. 


Including  only  works  directly  cited  in  the  text.    A  full  literature-list  is  given  in  the  works  of 
McClung  ('02,  2)  and  Montgomery  ('05). 


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STUDIES   ON   CHROMOSOMES 

III.  THE  SEXUAL  DIFFERENCES  OF  THE  CHROMO- 
SOME-GROUPS IN  HEMIPTERA,  WITH  SOME  CON- 
SIDERATIONS ON  THE  DETERMINATION  AND 
INHERITANCE  OF  SEX 


By 

EDMUND  B.  WILSON 


DIVISION  OF  GENETfCS 


REPRINTED   FROM 

THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY 

Volume   III 
No.  1 


BALTIMORE,  MD,,  U.   S.  A. 
February,   1906 


STUDIES  ON  CHROMOSOMES 

III.  THE  SEXUAL  DIFFERENCES  OF  THE  CHROMOSOME- 
GROUPS  IN  HEMIPTERA,  WITH  SOME  CONSIDERA- 
TIONS ON  THE  DETERMINATION  AND  INHERI- 
TANCE OF  SEX 

BY 

EDMUND  B.  WILSON 
WITH  Six  FIGURES 

Since  the  time  of  Henking's  able  paper  on  the  spermatogenesis 
of  Pyrrochoris  ('91),  it  has  been  known  that  in  certain  Hemiptera, 
and  in  some  other  insects,  a  dimorphism  exists  in  the  nuclear  con- 
stitution of  the  spermatozoa,  one-half  of  them  containing  the  so- 
called  "accessory"  or  " heterotropic "  chromosome,  while  in  the 
other  half  this  chromosome  is  lacking.  The  meaning  of  this  fact 
has  hitherto  remained  undetermined.  McClung  in  1902  devel- 
oped an  hypothesis  of  sex-production  based  on  the  conjecture  that 
the  heterotropic  chromosome  is  a  sex-determinant,  and  more 
specifically  that  spermatozoa  containing  this  chromosome  produce 
males,  for  the  very  obvious,  yet  fallacious,  reason  that  it  is  present 
in  the  male.  This  hypothesis  was  based  simply  on  the  fact 
that  the  spermatozoa  are  of  two  numerically  equal  classes,  like 
the  sexes  of  the  adults;  and  it  was  apparently  overthrown  by 
subsequent  observation.  The  hypothesis  implied  that  the  cells 
of  the  female  must  contain  one  chromosome  less  than  those  of 
the  male;  and  although  McClung  did  not  specifically  place  his 
assumption  in  this  form,  he  considered  it  extremely  improbable 
that  the  accessory  chromosome,  or  "any  such- element,"  is  present 
in  the  egg.  Sutton  ('02)  believed  that  he  had  found  a 
confirmation  of  this  in  the  grasshopper  Brachystola,  where  he 
showed  that  the  number  in  the  male  (spermatogonia)  is  twenty- 
three,  and  stated  that  in  the  female  (oogonia  and  follicle- 

JOURNAL  OF  EXPERIMENTAL  ZOOLOGY,  VOL.  in,  No.  i. 


2  Edmund  B.  Wilson 

cells)  the  number  is  twenty-two,  supporting  this  statement  by  a 
single  figure  (op.  cit.,  Fig.  n).  Sutton  was,  however,  able  to 
examine  only  a  very  few  of  the  female  groups,  and  the  object  is 
an  unfavorable  one  as  compared  with  the  Hemiptera,  owing  to 
the  less  compact  form  of  the  chromosomes.  McClung's  hypothe- 
sis seemed  to  be  rendered  completely  untenable  by  the  later  obser- 
vations of  Montgomery  on  Anasa  ('04),  and  of  Gross  on  Syromas- 
tes  ('04),  both  these  authors  describing  and  clearly  figuring  the 
same  number  of  chromosomes  (twenty-two)  in  the  male  and  the 
female  cells.  Gross  and  Wallace  ('05)  were  thus  independently 
led  to  the  conclusion  that  only  one  of  the  two  classes  of  spermat- 
ozoa was  functional,  namely,  that  in  which  the  heterotropic 
chromosome  is  present.  Those  of  the  other  class  were  assumed 
to  degenerate  after  the  fashion  of  polar  bodies. 

I  am  now  able  to  bring  forward  decisive  proof  that  the  appar- 
ently adverse  evidence  brought  forward  by  Montgomery  and 
Gross  was  based  on  errors  of  observation,  and  that  the  sexes  in 
Hemiptera  of  this  type  do  in  fact  show  a  constant  difference  in 
the  number  of  chromosomes.  As  far  as  these  animals  are  con- 
cerned, however,  McClung's  conjecture  as  to  the  mode  of  fertili- 
zation proves  to  have  been  the  reverse  of  the  truth;  for  it  is  the 
female,  not  the  male,  that  possesses  the  additional  chromosome, 
as  I  have  determined  beyond  all  doubt  in  four  genera,  namely, 
Anasa,  Alydus,  Harmostes  and  Protenor.  The  facts  leave  no 
doubt  that  both  forms  of  spermatozoa  are  functional;  that  all  of 
the  eggs  possess  the  same  number  of  chromosomes;  that  all  con- 
tain the  homologue,  or  maternal  mate,  of  the  accessory  or  hetero- 
tropic chromosome  of  the  male;  and  that  fertilization  by  sper- 
matozoa that  possess  this  chromosome  produces  females,  while 
males  are  produced  upon  fertilization  by  spermatozoa  that  do 
not  possess  it. 

A  second  type  of  dimorphism  of  the  nuclei  of  the  spermatozoa 
was  made  known  in  the  first  of  these  studies.  In  this  type  all 
of  the  spermatozoa  contain  the  same  number  of  chromosomes,  but 
half  of  them  contain  a  large  "idiochromosome"  and  the  other 
half  a  corresponding  small  one.  I  was  led  in  that  paper  to  suggest 
the  possibility  that  the  idiochromosomes  might  play  a  definite 


Studies  on  Chromosomes  3 

role  in  sex-production,  but  could  at  that  time  produce  no  evidence 
in  support  of  the  suggestion.  I  have  now  the  evidence  to  show 
that  this  suggestion  was  in  accordance  with  the  facts;  for  in  at 
least  four  genera,  Lygaeus,  Euschistus,  Coenus  and  Podisus,  both 
sexes  show  the  same  number  of  chromosomes,  but  the  small 
idiochromosome  is  present  only  in  the  male.  Somewhat  earlier, 
and  independently,  Stevens  ('05)  determined  a  precisely  similar 
fact  in  the  case  of  a  beetle,  Tenebrio,  which  indicates  that  the 
phenomenon  is  of  wide  occurrence  in  the  insects.  These  results 
confirm  the  correctness  of  my  conclusion  that  the  heterotropic 
or  "accessory"  chromosome  has  become  unpaired  in  the  male 
sex  through  the  disappearance  in  that  sex  of  its  mate,  and  give  a 
complete  explanation  of  the  fact  that  in  forms  possessing  the 
heterotropic  chromosome  the  male  number  is  odd  and  one  less 
than  the  female  number.  I  believe  that  these  facts  may  give 
the  basis  for  a  general  theory  of  sex-production. 

I.      DESCRIPTIVE 
A.     General  Character  of  the  Chromosome- groups 

In  two  preceding  papers  (Wilson,  '05,  i;  05,  3,)  (where  due 
acknowledgment  is  made  to  previous  observers  in  this  field) 
I  have  described  in  some  detail  the  general  nature  of  the  chro- 
mosomes in  these  insects.  For  such  an  investigation  as  the  present 
one,  the  Hemiptera  present  peculiar  advantages,  owing  above  all 
to  the  short  and  regular  form  of  the  chromosomes,  and  the  relative 
lack  of  crowding  in  the  equatorial  plate.  I  have  employed  almost 
exclusively  Flemming's  strong  fluid  as  a  fixative,  staining  the 
sections  with  iron-haematoxylin  and  extracting  until  the  cytoplasm 
is  nearly  or  quite  colorless.  The  best  preparations  thus  obtained 
leave  nothing  to  be  desired  in  point  of  brilliancy  and  clearness, 
and  show  the  chromosomes  with  a  distinctness  that  is  hardly 
exaggerated  by  the  black  and  white  figures  here  reproduced. 
The  very  large  number  of  sections  now  at  my  disposal  (including 
all  those  of  Paulmier  and  a  still  greater  number  of  new  prepara- 
tions of  my  own)  has  enabled  me  in  the  case  of  nearly  every 
species  to  examine  numerous  division-figures  (of  which  only  the 


4  Edmund  B.  Wilson 

best  have  been  selected  for  illustration)  and  to  satisfy  myself  thor- 
oughly of  the  constancy  of  the  relations  as  described.  Everyone 
familiar  with  such  objects  will,  however,  realize  that  in  regard  to 
such  matters  as  the  arrangement  and  size-differences  of  the 
chromosomes  certain  apparent  variations  appear  that  are  due  to 
slight  differences  in  the  form  and  position  of  the  chromosomes, 
and  to  the  various  degrees  of  foreshortening  thus  caused.  This 
introduces  a  slight  error,  into  both  the  observations  and  the  draw- 
ings, that  can  hardly  be  avoided.  A  second  source  of  error  lies 
in  the  degree  of  extraction,  which  produces  surprising  variations 
in  the  apparent  size  of  the  chromosomes — I  have  found,  for 
instance,  that  by  successive  extraction  the  chromosomes  may  be 
reduced  almost  to  one-half  their  original  apparent  size,  and  the 
smaller  chromosomes  may  thus  be  caused  almost  to  disappear  from 
view.  Camera  drawings  at  successive  stages  of  the  extraction  show, 
however,  that  the  relative  sizes  of  the  chromosomes  remain  sub- 
stantially unchanged,  and  the  comparison  of  the  same  object  after 
a  shorter  and  a  longer  extraction  has  thus,  in  a  number  of  cases, 
given  a  more  certain  result  than  could  otherwise  have  been 
obtained.  I  have,  whenever  it  was  possible,  figured  different 
stages  of  the  same  species  from  the  same  slide,  so  as  to  avoid  the 
error  due  to  different  degrees  of  extraction;  but  this  is  not  always 
possible,  since  as  a  rule  longer  extraction  is  required  to  give  a 
perfectly  clear  view  of  the  spermatogonial  groups  than  is  desirable 
for  the  spermatocyte-divisions.  For  the  comparison  of  the  two 
sexes,  different  slides  must  of  course  be  used,  and  to  this  is  due, 
I  am  sure,  some  of  the  size-differences  between  the  oogonial  and 
spermatogonial  groups  that  appear  in  the  figures. 

Making  all  due  allowance  for  the  sources  of  error  mentioned, 
it  remains  perfectly  clear  that  the  chromosomes  in  each  species 
show  among  themselves  constant  and  characteristic  size-differ- 
ences; and  further,  that  with  the  special  exceptions  in  the  male 
described  beyond,  the  chromosomes  of  the  unreduced  groups 
(/.  e.,  those  of  the  oogonia  and  spe'rmatogonia)  may  be  paired  off, 
two  by  two,  to  form  equal  or  symmetrical  pairs.  The  pairing  of 
the  chromosomes  is  most  evident  in  the  case  of  especially  small 
chromosomes  (such  as  the  m-chromosomes  of  Anasa,  Alydus, 


Studies  on  Chromosomes  5 

Harmostes,  etc.,  or  the  small  pair  of  ordinary  chromosomes  of 
Coenus  and  Euschistus,  described  beyond)  or  especially  large  ones 
such  as  the  largest  pair  in  Alydus,  and  in  some  of  the  species  of 
Euschistus.  Those  of  intermediate  size  are  also  obviously  paired 
in  some  of  the  forms  (e.  g.,  in  Protenor,  Fig.  i);  but  in  many  of 
the  species  the  several  pairs  are  not  sufficiently  marked  in  size  to 
admit  of  certain  recognition.  Nevertheless,  a  comparative  study 
of  many  species  has  convinced  me  of  the  correctness  of  the  con- 
clusion, first  indicated  by  Montgomery  ('01)  and  afterward  more 
fully  worked  out  by  Sutton  ('02),  that  all  the  chromosomes  (again 
with  the  special  exceptions  referred  to  above)  may  be  thus  paired, 
and  that  the  chromosome-group  as  a  whole  includes  two  parallel 
series  of  chromosomes  that  undoubtedly  represent  respectively 
the  descendants  of  those  that  originally  are  brought  together  in 
the  union  of  the  gametes.  This  is  very  clearly  brought  out  by 
making  camera  drawings  of  the  chromosomes,  and  arranging 
them  as  nearly  as  practicable  in  pairs  of  equal  size.  This  arrange- 
ment conspicuously  shows  the  sexual  differences,  as  may  be  seen 
by  a  comparison  of  Figs.  2,  /and  b  (Anasa)  and  5,  c  and  g  (Lygaeus). 
There  is,  of  course,  a  large  error  to  be  allowed  for  in  the  series 
as  thus  arranged,  and  no  pretense  to  complete  accuracy  in  the 
selection  of  the  members  of  most  of  the  pairs  can  be  made. 
Nevertheless,  when  all  due  allowance  for  differences  of  form, 
foreshortening  and  the  like  is  made,  the  fact  that  such  a  double 
series  exists  is  unmistakable.  When  it  is  borne  in  mind  that  the 
spermatid-nuclei  in  each  case  contain  a  single  series  of  chromo- 
somes showing  the  same  size-relations  (cf.  for  instance,  Figs.  I,  b, 
<:,  d;  2,  a,  d,  e;  3,  a,  e,  f;  4,  b,  /,  d,  /?),  it  becomes  in  a  high  degree 
probable  that  the  corresponding  pairs  of  the  somatic  groups  con- 
sist each  of  a  paternal  and  a  maternal  member,  in  accordance 
with  Montgomery's  original  and  fundamental  assumption  ('01). 
As  may  be  seen  by  a  comparison  of  the  figures,  the  members  of 
each  pair  when  in  their  natural  position,  do  not  as  a  rule  lie  in 
juxtaposition  but  may  occupy  any  relative  position.  Only  at 
the  period  of  synapsis  do  they  actually  couple,  two  by  two,  to 
form  the  bivalents  whose  members  are  subsequently  separated 
by  the  reducing  division. 


6  Edmund  B.  Wilson 

In  order  to  give  a  wider  basis  of  comparison  I  have  given  new 
figures  of  the  chromosome-groups  of  nearly  all  the  species, 
even  in  the  case  of  forms  already  figured  in  my  preceding  papers. 
Since  the  idiochromosomes  or  the  heterotropic  chromosome  form 
the  distinctive  differential  between  the  nuclei  of  the  two  sexes,  I 
shall  in  the  descriptive  part  of  this  paper  call  them  the  "differen- 
tial chromosomes." 

B.     First  Type.     Forms  Possessing  an  "Accessory"  or  Hetero- 
tropic Chromosome 

As  stated  above,  I  have  compared  the  males  and  females  in 
respect  to  the  chromosome-groups  in  four  genera,  selecting  for 
this  purpose  the  most  available  cells,  which  are  the  dividing 
oogonia  and  ovarian  follicle-cells  in  the  female,  the  spermato- 
gonia  and  investing  cells  of  the  testis-cysts  in  the  male.  The 
general  result  is  the  same  in  all,  but  owing  to  the  conspicuous 
size-difference  of  the  chromosomes  in  Protenor,  this  form  gives 
the  most  obvious  and  striking  evidence.1 

a.     Protenor  belfragei 

Montgomery  ('01)  first  made  known  the  general  character  of 
the  chromosome-groups  in  this  interesting  species,  showing  that 
the  spermatogonial  groups  show  an  odd  number,  thirteen,  that 
the  heterotropic  chromosome  (Montgomery's  "chromosome  #") 
is  immediately  recognizable  by  its  enormous  size — it  is  fully  twice 
the  size  of  the  largest  of  the  other  chromosomes — and  that  it  is 
unpaired  (though  he  considered  it  a  bivalent).  My  own  observa- 
tion confirms  his  description  in  every  point,  except  that  I  have 
never  seen  this  chromosome  transversely  constricted  into  two 
halves.  The  first  glance  at  a  good  preparation  of  the  spermat- 
ogonial metaphase,  as  seen  in  polar  view,  shows  this  huge  chro- 

^here  can  be  no  doubt  of  the  identification  of  the  follicle-cells;  but  there  is  some  uncertainty  regard- 
ing the  cells  here  called  oogonia,  since  they  are  from  the  undifferentiated  region  of  the  ovary  in  which  the 
distinction  between  oogonia  and  follicle-cells  cannot  be  made  out.  It  is  therefore  quite  possible  that  some 
of  the  groups  here  described  as  oogonia  may  be  from  very  young  follicle-cells  or  nutritive  cells;  but  this 
does  not  affect  the  main  result. 


Studies  on  Chromosomes  7 

mosomed  a  sa  long  worm-shaped  body  obviously  without  a  mate, 
(Fig.  i,  </-/).  The  remaining  twelve  chromosomes  may  be 
grouped  in  symmetrical  pairs  (indicated  by  numbers  in  Fig.  I, 


h 


// 


• 

>• 

•• 


A 


(' 


3 


*     lX  M 

**  !  ^  a-  *l  / 


5 


3          4 


FIGURE  il 


Protenor  belfragei. — a,  Anaphase  of  second  spermatocyte-division;  b,  c,  sister  groups,  from  the 
same  spindle,  polar  view,  second  spermatocyte-division;  d,  e,  /,  spermatogonial  groups;  g,  h,  groups 
from  immature  ovaries,  probably  oogonia;  i,  group  from  dividing  follicle-cell. 

d,  ^),  though  the  members  of  each  pair  may  occupy  any  relative 
position.  Of  these  six  pairs,  one  (2,  2)  is  always  much  larger  than 
the  others,  its  members  being  approximately  half  the  size  of  the 

JA11  the  figures  are  drawn  to  the  same  scale.  In  all,  h  denotes  the  heterotropic  chromosome, ;  the 
idiochromosomes  (large  and  small  in  some  cases  lettered  I  and  i  respectively),  m  the  paired  micro- 
chromosomes,  and  i  the  smallest  pair  of  ordinary  chromosomes. 


8  Edmund  B.  Wilson, 

heterotropic.  A  second  pair  (3,  3)  may  usually  be  distinguished 
as  the  next  largest,  and  a  third  pair  (7,  7)  as  the  smallest,  though 
this  is  not  always  obvious.  This  pair  probably  correspond 
to  the  "  m-chromosomes  "  of  my  preceding  paper.  The  remain- 
ing three  pairs  are  of  nearly  equal  size,  though  sometimes  they 
clearly  show  a  progressively  graded  series  as  in  Fig.  I,  </,  e.  In 
synapsis  the  six  paired  chromosomes  become  coupled,  as  usual, 
to  form  six  corresponding  bivalents,  while  the  large  chromosome 
remains  as  an  unpaired  univalent.  During  the  whole  growth- 
period  of  the  spermatocytes  this  chromosome  remains  in  a  con- 
densed spheroidal  state,  forming  a  very  large  chromosome- 
nucleolus.  In  the  prophases  of  the  first  division  it  again  elongates 
and  divides  longitudinally  in  this  division.  Each  secondary 
spermatocyte  accordingly  receives  seven  chromosomes.  In  the 
second  division  six  of  these  (the  products  of  the  bivalents)  again 
divide  equally,  while  the  seventh  (the  large  chromosome)  passes 
undivided  to  one  pole  (Fig.  I,  a).  One-half  of  the  spermatid 
nuclei  accordingly  receive  six  chromosomes,  the  other  half  seven, 
the  additional  one  being  the  large  heterotropic  chromosome 
(Fig.  i,  b,  c). 

In  the  female  the  chromosome-groups  of  the  dividing  oogonia 
and  follicle-cells  appear  with  a  clearness  not  inferior  to  that  shown 
in  the  spermatogonial  groups  (Fig.  I,  g-i).  It  is  at  once  apparent 
that  in  these  groups  there  are  two  very  large  chromosomes,  equal 
in  size,  in  place  of  the  single  one  that  appears  in  the  male,  while 
the  remaining  chromosomes  show  the  same  relations  as  in  the 
male.  There  are  accordingly  fourteen  chromosomes  in  all,  which 
may  be  equally  paired  off,  two  by  two,  and  no  chromosome  is 
without  a  mate  of  corresponding  size.  Since  the  largest  two  are 
of  the  same  relative  size  as  the  single  heterotropic  chromosome 
of  the  male,  it  is  quite  clear  that  one  of  them  must  have  been 
derived  from  a  spermatozoon  containing  this  chromosome,  while 
the  other  is  its  maternal  mate  or  homologue. 

I  have  not  been  able  to  follow  by  actual  observation  the  phe- 
nomena of  reduction,  maturation  and  fertilization  in  the  egg; 
but  the  data  are  sufficient  to  show,  with  a  degree  of  probability 
only  short  of  certainty,  what  must  be  the  history  of  the  chromo- 


Studies  on  Chromosomes  9 

somes  in  these  processes.  Since  the  oogonia  contain  fourteen 
equally  paired  chromosomes,  synapsis  in  the  oocyte  must  result 
in  the  formation  of  seven  symmetrical  bivalents — /.  e.,  seven 
couples  of  equal  chromosomes— and  each  egg  after  maturation 
contains  seven  univalent  chromosomes,  one  of  which  is  the 
maternal  representative  or  mate  of  the  heterotropic  chromosome 
of  the  male.  This  group  contains  one  chromosome  of  each  of  the 
original  pairs,  and  is  precisely  similar  to  the  group  present  in 
those  spermatozoa  that  contain  the  heterotropic  chromosome 
(Fig.  I,  <:).  Fertilization  by  such  a  spermatozoa  doubles  this 
group,  giving  the  condition  observed  in  the  female — /.  ^.,  fourteen 
chromosomes  equally  paired,  the  largest  pair  consisting  of  the 
heterotropic  chromosome  and  its  maternal  mate  (/,  J,  Fig.  I,  g,  h~). 
Fertilization  by  a  spermatozoon  that  lacks  the  heterotropic  chro- 
mosome will  give  the  condition  observed  in  the  male,  namely, 
thirteen  chromosomes,  of  which  twelve  are  equally  paired,  while 
the  thirteenth  is  the  large  unpaired  one  which  is  obviously  derived 
from  the  egg.  There  is  therefore  no  escape  from  the  conclusion 
that  both  forms  of  spermatozoa  are  functional,  that  females  are 
produced  upon  fertilization  by  spermatozoa  that  contain,  and 
males  upon  fertilization  by  spermatozoa  that  lack,  the  hetero- 
tropic chromosome.  Since  the  two  classes  of  spermatozoa  are 
equal  in  number,  fertilization  will  in  the  long  run  produce  males 
and  females  in  approximately  equal  numbers. 

b.     Anasa  tristis 

A  comparison  of  the  nuclei  of  the  two  sexes  in  this  species  gives 
a  precisely  concordant  result,  though  the  size-differences  do  not 
allow  of  so  exact  an  identification  of  the  differential  chromo- 
somes. In  the  preceding  study  I  showed  that  the  number  of 
chromosomes  in  the  male  (spermatogonia)  is  twenty-one,  not 
twenty-two  as  stated  by  previous  observers.  Study  of  the  sper- 
matogonial  metaphase  groups  shows  that  twenty  of  the  chro- 
mosomes may  be  equally  paired,  two  by  two,  while  the  remaining 
one  is,  of  course,  without  a  mate  (Fig.  2,  <?,  /).  The  unpaired 
heterotropic  chromosome  is  one  of  three  largest  chromosomes, 


IO  Edmund  B.  Wilson 

but  which  particular  one  cannot  be  determined  by  simple  inspec- 
tion, since  the  three  are  of  nearly  equal  size.  In  synapsis  two 
of  these  large  chromosomes  unite  to  form  the  largest  of  the  ten 

O  O 

bivalents  (/,  Fig.  2,  a)  that  appear  in  the  first  spermatocyte 
division.  The  third,  which  retains  its  compact  form  as  a  chro- 
mosome-nucleus during  the  growth-period,  remains  as  the  univ- 
alent  heterotropic  chromosome  (h,  Fig.  2,  a).  The  first  spermat- 
ocyte division  accordingly  shows  eleven  chromosomes,  ten  of 
which  are  bivalent,  and  one  (heterotropic)  is  univalent.  The 
distribution  of  these  chromosomes  in  the  maturation-division  takes 
the  usual  course,  the  heterotropic  chromosome  dividing  equally 
with  the  ten  bivalents  in  the  first  mitosis  while  its  products  pass 
undivided  to  one  pole  of  the  spindle  in  the  second  (Fig.  2,  6). 
Half  the  spermatozoa  accordingly  receive  ten  chromosomes,  one 
of  which  (/,  Fig.  2,  <:)  is  larger  than  the  others,  and  half  an  exactly 
similar  group  plus  the  large  heterotropic  chromosome,  or  eleven 
in  all  (Fig.  2,  J). 

The  oogonial  groups  show  invariably  twenty-two  chromosomes, 
which  may  be  arranged  in  eleven  equal  pairs  (Fig.  2,  g,  h).  In 
place  of  the  three  large  chromosomes  of  the  spermatogonial 
groups  appear  four  similar  chromosomes,  forming  two  equal 
pairs.  Two  of  these  four  are  obviously  the  large  chromosome, 
common  to  all  the  spermatozoa,  and  its  maternal  mate,  while 
the  other  two  must  be  the  heterotropic  chromosome  (derived  in 
fertilization  from  the  spermatozoon)  with  its  maternal  mate. 
It  is,  therefore,  clear  that  all  of  the  matured  eggs  must  contain 
eleven  chromosomes,  that  females  are  produced  upon  fertilization 
by  those  spermatozoa  that  contain  a  similar  group — /'.  <?.,  by  those 
containing  the  heterotropic — males  upon  fertilization  by  spermat- 
ozoa that  lack  the  heterotropic. 

The  ovarian  follicle-cells  often  show  chromosome-groups 
identical  with  those  of  the  oogonia  (Fig.  2,  /).  Not  infrequently, 
however,  the  number  of  chromosomes  is  much  greater,  and  the 
same  is  true  of  the  nuclei  of  the  investing  cells  of  the  ovary,  of 
the  oviduct  and  of  the  fat-body.  In  the  male  similar  multiple 
groups  are  not  uncommon  in  the  interstitial  and  investing  cells 
of  the  testis.  Only  in  a  single  case  have  I  succeeded  in  gaining 


Studies  on  Chromosomes 


ii 


H\ 


a 


'/  *   c 


Iff ••*•••• 


FIGURE  z 

Anasa  tristis. — a,  Metaphase  of  first  spermatocyte-division,  in  polar  view,  showing  the  nine  large 
bivalents  in  a  ring,  the  univalent  heterotropic  chromosome  below  it,  and  the  m-chromosome  bivalent 
in  the  center;  b,  anaphase  of  second  division;  c,  d,  sister-groups  from  the  same  spindle,  polar  view, 
second  division  (i  the  macrochromosome);  e,  spermatogonial  group;  /,  the  same  chromosomes  arranged 
in  pairs;  g,  obgonial  group  from  a  larva;  h,  the  same  group  arranged  in  pairs;  /,  spermatogonial  group; 
j,  group  from  a  dividing  follicle-cell;  k,  double  group,  from  a  cell  toward  the  periphery  of  a  larval  ovary. 


12  Edmund  B.  Wilson 

a  clear  and  complete  view  of  such  a  group;  but  this  one  case 
suffices  to  give,  with  great  probability,  the  explanation  of  the 
increased  number  of  chromosomes.  In  this  case  every  chro- 
mosome of  the  metaphase  group  may  be  clearly  seen,  and  the 
number  is  exactly  twice  the  oogonial  number,  namely,  forty-four 
(Fig.  2,  £).  Careful  study  clearly  shows  that  this  group  contains 
four  microchromosomes  and  eight  macrochromosomes,  in  each 
case  twice  the  number  of  those  present  in  the  oogonia.  This 
leaves  no  doubt  that  in  this  case  all  the  chromosomes  have  divided 
once  without  the  occurrence  of  a  cytoplasmic  division,  and  makes 
it  probable  that  the  increase  in  number  in  the  cells  in  question  is 
always  due  to  a  process  of  this  kind.  I  have  not  been  able  to 
obtain  faultless  preparations  of  the  dividing  cells  of  other  tissues, 
and  can  only  state  that  in  the  ectodermal  cells  of  the  larva  the 
number  of  chromosomes  is  approximately  the  same  as  in  the 
oogonia.  The  multiple  chromosome-groups  were  only  observed 
in  the  cells  mentioned  above,  all  of  which,  it  may  be  observed, 
are  degenerating  or  highly  specialized  cells. 

c.     Alydus  pilosulus 

Despite  the  small  number  of  chromosomes  (  9  14,  d1  13,  as  in 
Protenor)  this  genus  is  in  some  respects  less  favorable  for  detailed 
analysis  than  either  of  the  ones  described  above,  for  the  size  of 
the  heterotropic  chromosome  does  not  distinguish  it  sufficiently  from 
the  other  chromosomes  to  allow  of  its  certain  identification  in  the 
spermatogonia.  The  main  fact  appears,  however,  as  clearly  as 
in  Protenor  or  Anasa  that  the  female  has  one  more  chromosome 
than  the  male. 

In  polar  views  of  the  second  spermatocyte-division  this  species 
shows  the  sister  spermatid-groups  with  great  beauty,  one  having 
six  chromosomes  and  one  seven  (Fig.  3,  ^,  /).  These  chromo- 
somes show  at  least  five  distinguishable  sizes  that  are  constant, 
namely,  (i)  a  largest;  (2)  an  extremely  small  one  (w-chromo- 
some);  (3)  a  second  smallest  (the  heterotropic);  (4)  a  second 
largest,  and  (5)  three  others  intermediate  in  size  between  (3)  and 
(4),  one  of  which  is  frequently  a  little  larger  than  the  other  two. 


Studies  on  Chromosomes  13 

The  sister  groups  are  practically  exact  duplicates  save  for  the 
heterotropic  which  varies  considerably  in  appearance  as  seen  from 
the  pole  owing  to  foreshortening  (cf.  the  side-views  given  in  my 
preceding  paper).  The  spermatogonia  correspondingly  show 
always  thirteen  chromosomes  (Fig.  3,  <?),  of  which  the  largest  and 
the  smallest  pair  are  at  once  distinguishable.  Next  follow  four 
chromosomes  nearly  equal  in  size,  two  of  them  often  appreciably 
smaller  than  the  other  two.  Of  the  remaining  five,  one  must  be 
the  unpaired  heterotropic;  but,  as  already  stated,  it  cannot  be 
positively  identified  by  inspection.  Closely  similar  groups  may 


^^  m 

4a*  ** 

ft*. 


a 


k 

•. 


c      ?  d  f 


J 

FIGURE  3 

Alydus  pilosulus. — a,  Spermatogonial  group;  b,  group  from  a  dividing  investing  cell  of  the  testis; 
c,  oogonial  group;  d,  from  a  dividing  cell  of  an  egg-follicle;  e,  f,  two  pairs  of  sister-groups,  each  from  a 
single  spindle,  anaphase  of  second  spermatocyte-division,  in  polar  view. 

occasionally  be  found  in  dividing  cells  of  the  enveloping  cells  of 
the  testis  (Fig.  3,  &).  Whether  multiple  groups  occur  like  those 
described  in  Anasa,  I  cannot  say. 

The  dividing  oogonia  and  follicle-cells,  of  which  a  large  number 
have  been  observed,  always  show  fourteen  chromosomes  that  may 
be  arranged  in  seven  equal  pairs  (Fig.  3,  c,  d}.  As  in  the  sper- 
matogonia, the  largest  and  the  smallest  pair  are  usually  at  once 
recognizable,  and  also  the  four  second  largest.  The  remaining 
six,  of  nearly  equal  size,  must  of  course  include  the  heterotropic 
chromosome  and  its  maternal  mate. 


14  Edmund  B.  Wilson 

d.     Harmostes  reflexulus 

My  material  of  this  species  is  much  less  abundant  than  that  of 
the  three  preceding,  and  the  preparations  are  not  of  the  same 
excellence.  They  nevertheless  show  beyond  doubt  that  the  num- 
bers are  here  the  same  as  in  Protenor  and  Alydus,  viz.,  thirteen  in 
the  male  and  fourteen  in  the  female.  In  my  sections  of  both  sexes 
the  chromosomes  appear  less  regular  in  contour  than  in  the  other 
species  examined  (probably  owing  to  somewhat  defective  fixation). 
They  show  clearly,  however,  in  both  sexes  a  largest  pair  and  a 
smallest  (m-chromosomes),  as  in  the  other  forms. 

C.      Second    Type.     Forms  Possessing  Unequal  Idiochromosomes 

The  sexual  differences  of  these  forms  have  been  worked  out  in 
Lygaeus  turcicus,  five  species  of  Euschistus  (variolarius,  ictericus, 
tristigmus,  fissilis  and  servus),  Coenus  delius  and  Podisus  spinosus. 
In  the  last  named  species  the  unreduced  number  is  sixteen,  in  the 
others  fourteen.  In  all,  the  number  of  chromosomes  is  the  same 
in  both  sexes,  but  while  the  males  show  a  large  and  a  small  idio- 
chromosome,  the  females  show  two  large  idiochromosomes  that 
are  equally  paired.  This  difference  clearly  appears  in  all  the 
species  examined  but  is  most  conspicuous  in  Euschistus  vario- 
larius, E.  ictericus  and  Lygaeus  turcicus,  where  the  inequality 
of  the  idiochromosomes  is  most  marked.  The  relative  size  of 
the  idiochromosomes  varies  somewhat  (perhaps  owing  to  differ- 
ences in  the  degree  of  extraction  of  the  dye)  but  on  the  whole  is 
characteristic  of  the  different  species,  as  described  below. 

In  all  of  the  species  of  Euschistus  examined,  and  in  Ccenus 
delius,  a  largest  and  a  smallest  pair  of  ordinary  chromosomes 
(the  latter  marked  s  in  some  of  the  figures)  are  readily  distinguish- 
able. These  give  rise  to  corresponding  large  and  small  bivalents 
in  the  first  mitosis,  and  are  recognizable  as  single  chromosomes 
in  the  spermatid-groups  (Figs.  4,  5).  The  small  chromosomes 
are  in  every  case  smaller  than  the  large  idiochromosome,  and  in 
Mineus  bioculatus  (Fig.  4,  /?,  q)  are  actually  smaller  than  the  small 
idiochromosome.  It  is  possible  -that  this  pair  of  chromosomes 


Studies  on  Chromosomes 


FIGURE  4 

Euschistus,  Mineus. — a,  E.  variolarius,  second  spermatocyte-division;  b,  sister-groups,  second 
division;  c,  d,  corresponding  views  of  E.  servus;  e,  second  spermatocyte-division,  E.  tristigmus;  /,  g, 
E.  variolarius,  spermatogonial  and  oogonial  groups  respectively;  h,  i,  corresponding  views  of  E.  servus; 
j,  k,  the  same,  E.  ictericus;  /,  m,  the  same,  E.  tristigmus;  n,  o,  the  same,  E.  fissilis;  p,  Mineus  bioculatus, 
second  spermatocyte-division;  q,  sister-groups,  from  the  same  spindle,  second  division. 


1 6  Edmund  B.  Wilson 

may  correspond  to  the  microchromosomes,  or  ra-chromosomes, 
that  are  so  characteristic  of  the  first  type  (m,  in  Figs.  2,  3). 

e.     Euschistus 

In  E.  variolarius  the  inequality  of  the  idiochromosomes  (Fig.  4, 
a)  is  greater  than  in  any  other  of  the  observed  forms  excepting 
Lygaeus  turcicus.  The  sister  spermatid-groups  (Fig.  4,  &)  consist 
in  each  case  of  a  ring  of  six  ordinary  chromosomes  with  the  idio- 
chromosome  near  its  center.  In  the  outer  ring  may  be  distin- 
guished as  a  rule  four  or  five  different  sizes  of  chromosomes,  the 
largest  and  smallest  (/)  being  always  recognizable,  and  usually 
also  a  second  largest  and  second  smallest.  The  large  idiochro- 
mosome  is  always  distinctly  larger  than  the  smallest  chromosome 
(/)  of  the  outer  ring,  while  the  small  idiochromosome  is  very  much 
smaller  than  either,  and  in  long  extracted  preparations  looks 
exactly  like  a  centrosome.  The  spermatogonial  groups  corre- 
spondingly show  seven  pairs  of  chromosomes  (Fig.  4,  /),  of  which 
the  small  idiochromosome,  the  smallest  pair  of  ordinary  chro- 
mosomes, and  two  large  pairs  are  recognizable.  The  remaining 
seven  include  three  equal  pairs,  while  the  seventh  is  the  large 
idiochromosome,  but  it  is  impossible  to  identify  this  chromosome 
more  nearly.  The  oogonial  groups  show  fourteen  equally  paired 
chromosomes,  as  shown  in  Fig.  4,  g;  but  my  preparations  do  not 
show  this  so  well  in  this  species  as  in  the  others. 

E.  ictericus  shows  a  similar  spermatogonial  group  (Fig.  4,  ;) 
except  that  the  small  idiochromosome  is  relatively  a  little  larger 
and  the  small  pair  of  ordinary  chromosomes  but  slightly  smaller 
than  the  others.  The  oogonial  groups  (Fig.  4,  k,  an  unusually 
open  specimen)  very  clearly  show  the  absence  of  the  small  idio- 
chromosome, but  the  equal  pairing  of  the  chromosomes  is  less 
obvious  than  in  the  following  species. 

In  E.  tristigmus  (Fig.  4,  e,  /,  m)  the  small  idiochromosome  is 
relatively  much  larger  than  in  the  foregoing  species,  while  in 
E.  servus,  it  is  usually  a  little  larger  still  (Fig.  4,  c,  d,  h).  In  both 
these  forms  the  smallest  pair  of  ordinary  chromosomes  are  at  once 
recognizable  in  the  spermatogonia  (j,  Fig.  4,  h,  I)  and  the  equal 
pairing  of  the  others  is  evident.  In  E.  servus  the  oogonial  groups 


Studies  on  Chromosomes  17 

show  the  equal  pairing  of  all  the  chromosomes  with  equal  clear- 
ness, the  absence  of  the  small  idiochromosome  being  evident 
(Fig.  4,  /).  The  small  pair  (/)  evidently  correspond  to  the  small 
pair  in  the  male  (4,  />)  and  the  large  idiochromosome-pair  must 
therefore  be  represented  by  one  of  the  larger  pairs.  Fig.  4,  n,  o, 
show  the  spermatogonial  and  oogonial  groups  of  E.  fissilis,  show- 
ing the  same  relations  as  in  E.  servus,  save  that  the  small  pair  are 
relatively  larger. 

The  above-described  species  of  Euschistus,  while  agreeing  pre- 
cisely in  the  general  relations,  present  individual  differences  so 
marked  as  to  show  that  even  the  species  of  a  single  genus  may  be 
distinguishable  by  the  chromosome-groups.  In  this  case  the 
most  interesting  feature  is  the  series  shown  in  the  inequality 
of  the  idiochromosomes,  which  becomes  progressively  greater 
in  the  series  (i)  E.  servus,  (2)  tristigmus,  fissilis,  (3)  ictericus, 
(4)  variolarius,  the  inequality  in  the  last  case  being  fully  as  great 
as  in  Lygaeus.  I  may  again  mention  the  fact  that  in  the  opposite 
direction  the  genus  Brochymena  often  shows  the  idiochromosomes 
less  unequal  than  in  E.  servus;  in  Mineus  they  are  sometimes 
of  nearly  equal  size  (Fig.  4,  />,  </),  while  in  Nezara  no  inequality 
exists.  Practically  all  intermediate  conditions  are  therefore  shown 
within  the  limits  of  a  single  family  between  the  extreme  inequality 
shown  in  E.  variolarius  and  no  inequality  at  all.  It  is  quite  clear 
from  the  observations  here  brought  forward  that  this  progressive 
differentiation  has  occurred  only  in  the  male  sex,  as  I  conjectured 
in  my  first  paper. 

/.     Coenus  delius 

The  relations  in  this  form  are  so  closely  similar  to  those  seen 
in  Euschistus  servus  or  fissilis,  as  described  above,  as  hardly  to 
require  separate  description.  Fig.  5,  b,  shows  the  spermatogonial 
metaphase-group;  5,  i,  the  corresponding  oogonial  group.  Both 
these  preparations  show  very  clearly  the  small  pair  (j)  of  ordinary 
chromosomes  (not  so  well  shown  in  the  figure  of  the  spermat- 
ogonial group  in  my  first  paper).  Here,  as  in  Euschistus,  it  is 
evident  that  the  large  idiochromosome  is  much  larger  than  the 
members  of  the  small  pair. 


18  Edmund  B.  Wilson 

g.     Lygaeus  turcicus 

In  this  species  the  inequality  of  the  idiochromosomes  is  nearly 
or  quite  as  great  as  in  Euschistus  variolarius,  but  the  differentia- 
tion of  the  chromosome-pairs  is  less  marked  than  in  that  species, 
and  the  small  pair  cannot  be  distinguished  with  certainty  in  any 
of  the  stages.  In  the  spermatogonial  groups,  accordingly,  only 
the  small  idiochromosome  is  markedly  smaller  than  the  others 
(Fig.  5,  c,  </);  and  hence  its  lack  of  an  equal  mate  is  rendered  very 
conspicuous.  In  the  female  the  small  idiochromosome  is  absent 
as  usual  and  all  the  chromosomes  are  equally  paired  (Fig.  5,  /,  ^). 
The  idiochromosomes  cannot  be  distinguished  from  the  ordinary 
chromosomes. 

b.     Podisus  spinosus 

In  this  species  both  sexes  show  sixteen  chromosomes.  In  the 
spermatogonial  groups  (of  which  I  am  now  able  to  give  a  better 
figure  than  the  one  in  my  first  paper)  the  small  idiochromosome 
appears  relatively  larger  than  in  any  of  the  foregoing  species, 
though  still  not  more  than  half  the  size  of  any  of  the  others 
(Fig.  5,  y).  In  the  female  (follicle-cells,  Fig.  5,  k)  all  the  chro- 
mosomes are  equally  paired  and  the  small  idiochromosome  is 
absent,  but  owing  to  the  relatively  large  size  of  the  latter  in  the 
male  the  chromosome-groups  of  the  two  sexes  do  not  show  so 
obvious  a  contrast  as  in  the  foregoing  cases. 

Resume  and  Conclusions  Regarding  the  Second  Type 

In  all  the  forms  described  under  this  type  the  two  sexes  show 
the  same  number  of  chromosomes  but  differ  in  that  the  male 
groups  include  a  large  and  a  small  idiochromosome  while  the 
female  groups  have  two  large  idiochromosomes  of  equal  size. 
This  result  agrees  with  that  already  reached  by  Stevens  ('05)  in 
the  case  of  the  beetle  Tenebrio,  and  involves  the  same  conclusions 
that  she  has  indicated.  Since  all  the  chromosomes  of  the  oogonial 
groups  are  equally  paired,  it  is  evident  that  all  the  matured  eggs 
must  contain  half  such  a  group,  one  of  the  chromosomes  being 
the  maternal  representative,  or  mate,  of  the  large  idiochromosome 


Studies  on  Chromosomes 


/Ik 

w 

w 

14 


f  •. 


o"  C 


If  I  I  ••• 
It  M  M 

d  <?  e 


•  I  «  t  «  M 


FIGURE  5 

Lygaeus,  Coenus,  Podisus,  Nezara. — a,  Lygaeus  turcicus,  second  spermatocyte-division;  b,  sister- 
groups,  second  division;  c,  d,  spermatogonial  groups;  e,  the  chromosomes  of  d  arranged  in  pairs; 
/,  oSgonial  group;  g,  the  same  in  pairs;  h,  i,  Coenus  delius,  spermatogonial  and  follicle-cell  groups; 
/,  m,  Nezara  hilaris,  spermatogonial  and  oogonial  groups  respectively. 


2O  Edmund  B.  Wilson 

of  the  male.  Fertilization  of  such  an  egg  by  a  spermatozoon  con- 
taining the  small  idiochromosome  will  produce  a  group  identical 
with  that  occurring  in  the  male;  fertilization  by  one  containing 
the  large  idiochromosome  will  produce  the  characteristic  female 
group.  This  result  is  thoroughly  consistent  with  that  obtained 
in  the  first  type;  for  if  the  small  idiochromosome  be  supposed  to 
disappear  in  the  male,  the  phenomena  become  in  every  respect 
identical  with  those  occurring  in  the  first  type.  The  large  idio- 
chromosome is  therefore  undoubtedly  homologous  with  the 
heterotropic  chromosome,  and  the  latter  owes  its  unpaired 
character  to  the  fact  that  its  former  paternal  mate  has  vanished, 
as  I  conjectured  in  my  first  paper. 

It  is  further  evident  that  in  synapsis,  in  both  sexes,  the  members 
of  each  chromosome-pair  become  coupled  to  form  symmetrical 
bivalents,  except  in  case  of  the  idiochromosomes  of  the  male. 
In  this  case  alone  do  chromosomes  of  unequal  size  couple  to  form 
an  asymmetrical  bivalent;  and  it  is  a  consequence  of  this  coupling 
that  the  subsequent  distribution  allots  the  small  idiochromosome 
to  one-half  of  the  spermatozoa  and  the  large  one  to  the  other  half. 

D.     Third    "Type.      Forms    in   which    the   Idiochromosomes    are 

of  Equal  Size 

Of  these  forms  I  have  been  able  to  examine  only  a  single  case, 
namely,  that  of  Nezara  hilaris;  and  in  the  course  of  a  whole 
summer's  collecting  I  obtained  but  a  single  female  in  the  proper 
stage  to  show  the  oogonial  divisions.  Fortunately  both  ovaries 
show  a  considerable  number  of  division-figures  which  demonstrate 
the  facts  with  perfect  clearness. 

A  particular  interest  attaches  to  this  form  on  account  of  the 
fact,  described  in  my  first  paper,  that  the  idiochromosomes  are  of 
equal  size  and  hence  give  no  visible  differential  between  the  two 
classes  of  spermatozoa.  This  form  gives  therefore  a  test  case  con- 
cerning my  general  conclusion  that  the  differentiation  of  the 
idiochromosomes  has  occurred  only  in  the  male;  for  since  these 
chromosomes  are  here  alike  in  all  the  spermatozoa,  it  might  with 
some  plausibility  be  assumed  that  the  differentiation  had  in  this 


Studies  on  Chromosomes  21 

species  taken  place  in  the  female.  The  facts  conclusively  show 
that  such  is  not  the  case. 

The  spermatogonial  groups  (Fig.  5,  /)  show  fourteen  chromo- 
somes, all  of  which  may  be  symmetrically  paired.  The  smallest 
pair,  /',  /,  (as  I  showed  in  my  first  paper)  are  the  idiochromosomes 
as  is  shown  by  their  characteristic  behavior  during  the  growth- 
period  and  in  the  maturation-divisions.  In  synapsis  the  twelve 
larger  chromosomes  couple  to  form  six  bivalents,  while  the  idio- 
chromosomes divide  as  separate  univalents  in  the  first  spermat- 
ocyte-division.  Their  products  then  conjugate  as  usual  to  form 
the  idiochromosome-dyad,  which  differs  from  all  the  forms  hitherto 
observed  in  being  composed  of  two  equal  members.  All  the 
spermatid-nuclei  are  accordingly  exactly  similar  in  appearance 
and  no  visible  dimorphism  exists  (cf.  Fig.  4  of  my  first  paper, 
Wilson,  '05,  i).  We  should  accordingly  expect  to  find  the 
oogonial  groups  exactly  similar  to  the  spermatogonial;  and  such 
is  clearly  shown  to  be  the  fact  by  the  preparations,  the  oogonial 
groups  showing  fourteen  equally  paired  chromosomes  among 
which  the  idiochromosomes  are  readily  recognizable  by  their 
small  size  (Fig.  5>  m)- 

In  this  case,  therefore,  alone  among  all  those  examined,  no 
visible  differences  are  shown  by  the  nuclei  of  the  two  sexes. 
One  pair  of  the  chromosomes  are,  however,  different  in  nature 
from  the  others,  as  is  shown  by  their  different  behavior  in  the 
male  in  the  growth-period  and  in  synapsis;  and  it  is  quite  clear 
that  the  two  members  of  this  pair  are  always  assigned  to  different 
spermatozoa.  In  respect  to  this  chromosome,  therefore,  the 
spermatozoa  fall  into  two  classes  as  truly  as  the  other  forms, 
though  they  cannot  be  distinguished  by  the  eye.  It  is  hardly 
necessary  to  point  out  how  important  this  case  is  in  giving  a  firm 
basis  of  comparison  with  the  more  usual  forms  in  which,  if  we  can 
trust  the  existing  accounts,  all  of  the  functional  spermatozoa  are 
exactly  alike  in  appearance,  and  no  sexual  differences  of  the  chro- 
mosome-groups are  apparent. 


22  Edmund  B.  Wilson 

E.     The  Differential  Chromosomes  in  the  Synaptic  and  Growth- 
periods 

I  will  now  briefly  consider  a  very  marked  difference  between 
the  sexes  in  respect  to  the  behavior  of  the  differential  chromosomes 
during  the  contraction-phase  of  synapsis  and  the  succeeding  early 
growth-period.1  In  the  male,  as  was  fully  described  in  my  last 
paper,  both  the  heterotropic  chromosome  and  the  idiochromo- 
somes  condense  early  in  the  growth-period  (usually  as  early  as  the 
contraction-phase  of  synapsis)  to  form  rounded,  condensed, 
intensely-staining  chromosome-nuclei.  In  this  condition  they 
persist  throughout  the  whole  growth-period  of  the  spermatocyte, 
without  ever  assuming  the  looser  texture  and  more  elongate  form 
of  the  other  chromosomes.  In  the  earlier  part  of  this  period  they 
are  as  a  rule  closely  associated  with  a  large  pale  plasmosome,  but 
later  become  separated  from  it. 

In  the  female  no  trace  of  such  a  chromosome-nucleus  can  be 
found  in  the  contraction-figure  of  the  synaptic  period.  My  best 
preparations  of  this  stage  are  from  the  ovaries  of  the  larval  Anasa, 
which  show  a  distinct  synaptic  zone  of  oocytes  intervening  between 
the  zone  of  multiplication  and  the  growth-zone;  but  I  have 
observed  the  same  condition  in  the  ovaries  of  recently  emerged 
adults  of  Harmostes,  Alydus,  Euschistus,  Coenus  and  Podisus. 
In  all  these  forms  the  contraction-figure  is  very  similar  to  that  of 
the  spermatocytes,  the  chromosomes  being  in  the  form  of  deeply 
staining,  ragged,  and  apparently  longitudinally  split  loops  that 
are  crowded  into  a  spheroidal  mass  toward  the  center  or  one  side 
of  the  nucleus  and  surrounded  by  a  large  clear  space.  The  nuclei 
at  this  time  occasionally  show  one  or  two  small  deeply-staining 
nucleolus-like  bodies  (probably  plasmosomes);  but  these  are 
much  smaller  than  the  chromosome-nuclei  of  the  spermatocytes 
at  this  period,  and  in  many  of  the  nuclei  are  absent.  The  contrast 
between  these  nuclei  and  those  of  the  male  at  the  corresponding 
period  is  so  striking  as  to  be  at  once  apparent.  In  later  stages  the 
chromosomes  spread  through  the  nuclear  cavity,  become  looser 
in  texture  and  finally  give  rise  to  a  fine  reticular  structure.  In 

JA  fuller  presentation  of  observations  on  these  phenomena  is  reserved  for  a  subsequent  paper. 


Studies  on  Chromosomes  .  23 

these  stages  a  variable  number  of  deeply-staining  nucleoli  make 
their  appearance;  but  their  true  nature  can  only  be  determined 
positively  when  the  whole  ovarian  life  of  the  egg  has  been  followed 
and  the  process  of  maturation  observed.  I  can,  therefore,  only 
state  that  no  chromosome-nucleolus  is  present  in  the  contraction 
period  of  synapsis,  or  in  the  early  growth-period;  and  even  though 
it  be  present  in  later  stages,  which  I  think  is  very  doubtful,  a  wide 
difference  between  the  sexes  would  still  exist  in  respect  to  the 
earlier  period. 

F.     General  Resume 

The  foregoing  results  may  be  given  a  general  formulation  as 
follows :  If  n  be  the  unreduced  number  of  chromosomes  in  the 
female,  the  matured  eggs  in  all  cases  contain  half  this  number  ("). 
The  males  are  of  three  types.  In  the  first,  one  of  the  chromo- 
somes (the  heterotropic  or  "accessory")  is  without  a  mate,  and 
the  unreduced  number  is  accordingly  one  less  than  that  of  the 
female.  Half  the  spermatozoa  possess,  and  half  lack,  the  hetero- 
tropic chromosome,  the  first  class  having  the  same  number  as  the 
matured  eggs  ("),  the  second  class  one  less  (5-1).  In  the  second 
type  the  male  has  the  same  number  of  chromosomes  as  the  female, 
but  possesses  one  large  and  one  small  idiochromosome  while  the 
female  possesses  two  large  ones.  In  maturation  half  the  spermat- 
ozoa receive  the  small  and  half  the  large  idiochromosome.  The 
third  type  differs  from  the  second  in  that  the  idiochromosomes  are 
of  equal  size  in  both  sexes,  and  no  visible  differences  exist  between 
the  two  classes  of  spermatozoa  or  the  somatic  groups  of  the  two 
sexes.  Designating  the  large  and  small  idiochromosomes  as  7 
and  /  respectively,  the  relations  in  fertilization  and  sex-production 
are  as  follows: 

TYPE  I 

(PROTENOR,  ANASA,  ALYDUS,  HARMOSTES) 

Egg  "  +  spermatozoon  ^  (including  heterotropic)      =  n  (female). 
Egg  5  +  spermatozoon  ™  —  i  (heterotropic  lacking)  =  n  —  I  (male). 

TYPE  II 

(LYGAEUS,  EUSCHISTUS,  COENUS,  PODISUS) 

Egg  5  (including  /)  +  spermatozoon  ^  (including  7)  =  n  (including  //)  (female). 
Egg  ^  (including  7)  +  spermatozoon  ™  (including  »')  =  n  (including  /»')  (male). 


24  t     Edmund  B.  Wilson 

TYPE  III 

(NEZARA) 

Egg  ™  +  spermatozoon  ™  =  n  (male  or  female,  including  in  each  case  two  equal  idiochromosomes). 

These  relations  are  graphically  shown  in  the  following  diagram 
(Fig.  6)  in  which  the  differential  chromosomes  are  black  and  the 
ordinary  ones  unshaded  (only  two  pairs  of  the  latter  shown).  For 
the  sake  of  simplicity  only  the  final  result  of  synapsis  (second 
column)  and  the  ensuing  process  of  reduction  (third  column)  are 
shown,  without  regard  to  variations  of  detail.  The  matured  eggs 
(ov)  are  represented  with  a  single  polar  body  (the  result  of  the 
reduction-division)  which  is  greatly  exaggerated  in  size.  The 
female-producing  and  male-producing  spermatozoa  (sp}  are 
lettered  a  and  b  respectively.  It  will  be  evident  from  an  inspection 
of  this  diagram  that  the  second  type  may  readily  be  derived  from 
the  third,  and  the  first  from  the  second  by  the  reduction  (second 
type)  and  final  disappearance  (first  type)  of  one  of  the  differential 
chromosomes.  This  I  believe  to  represent  the  actual  relations 
of  the  three  types. 

II.       GENERAL. 

In  recent  years  evidence  has  steadily  accumulated  to  strengthen 
the  view  that  the  general  basis  of  sex-production  is  given  by  a 
predetermination  existing  at  least  as  early  as  the  fertilized  egg, 
but  there  is  a  wide  divergence  of  opinion  in  regard  to  the  condi- 
tions preexisting  in  the  gametes  prior  to  their  union.1 

The  fact  that  in  some  organisms  (such  as  Dinophilus,  Hyda- 
tina  or  Phylloxera)  the  unfertilized  eggs,  sometimes  even  in  the 
ovary,  are  visibly  distinguishable  as  male-producing  and  female- 
producing  forms,  has  led  a  number  of  recent  writers  to  deny  that 
the  spermatozoon  can  play  any  part  in  sex-determination.  Beard, 
for  example,  asserts  that  "The  male  gamete,  the  spermatozoon, 
has  and  can  have  absolutely  no  influence  in  determining  the  sex 

'The  general  question  of  sex-determination,  with  its  literature,  has  within  the  past  five  years 
been  so  ably  and  thoroughly  reviewed  by  Cue'not,  Strasburger,  Beard,  von  Lenhosse'k,  O.  Schultze 
and  others,  that  I  shall  here  limit  myself  in  the  main  to  an  analysis  of  the  new  observations  brought 
forward. 


Studies  on  Chromosomes  . 


OOGONIA  FERTILIZATION 

AND  SYNAPSIS     / A 

SPERMATOGONIA  Gametes  Spermatozoa  Zygotes 


/        Pro  tenor 


I 


///  Nezara 


Fio.  6. 


26  Edmund  B.  Wilson 

of  the  offspring"  ('02,  p.  712);  and  a  similar  conclusion,  though 
less  dogmatically  stated,  is  reached  in  the  general  reviews  of 
Lenhossek  ('03)  and  O.  Schultze  ('03).  The  opposite  view  that 
the  spermatozoon  alone  is  concerned  in  sex-determination  (which 
like  the  preceding  one,  is  of  very  ancient  origin)  has,  however,  been 
maintained  by  some  recent  writers,  for  instance,  Block  (whose 
work  I  know  only  from  Cuenot's  review)  and  McClung,  as  already 
mentioned.1  On  the  other  hand,  both  Cuenot  ('99)  and  Stras- 
burger  ('oo)  in  their  able  reviews,  have  argued  that  both  gametes 
may  be  concerned  in  sex-determination;  and  the  last  named 
author  urged  the  view,  afterward  recognized  as  probable  by 
Bateson  and  developed  in  detail  by  Castle  ('03),  that  sex-produc- 
tion takes  place  in  accordance  with  the  Mendelian  principles  of 
inheritance. 

The  observations  here  brought  forward,  together  with  those  of 
Stevens  on  Tenebrio,  establish  the  predestination  (in  a  descriptive 
sense)  of  two  classes  of  spermatozoa,  equal  in  number,  as  male- 
producing  and  female-producing  forms.  Though  indistinguish- 
able to  the  eye  in  their  mature  state,  these  two  classes  differ  visibly 
in  nuclear  constitution  at  the  time  of  their  formation;  and  since 
this  occurs  in  the  same  order  of  insects  as  Phylloxera,  where  the 
eggs  are  visibly  distinguishable  (by  their  size)  as  male-producing 
and  female-producing  forms,  it  is  evident  that  a  substantial  basis 
now  exists  for  the  views  expressed  by  Cuenot  and  Strasburger,  and 
for  the  Mendelian  interpretation  of  sex-production  worked  out 
by  Castle.  Whether  in  the  Hemiptera  that  form  the  subject  of 
this  paper  the  eggs  are,  like  the  spermatozoa,  predestined  as  male- 
producing  and  female-producing  forms  can  at  present  be  a  matter 
of  inference  only.  I  have  not  been  able  to  distinguish  such  classes 
by  their  size,  and  the  data  show,  almost  with  certainty,  that  if 
they  exist  they  do  not  exhibit  any  visible  nuclear  differences 
like  those  present  in  the  spermatozoa.  But  this  gives  no  ground 
for  denying  their  existence.  No  visible  nuclear  dimorphism  of 

J"By  exclusion  then,  it  would  seem  that  the  determination  of  this  difference  (the  sexual  one)  is 
reposed  in  the  male  element"  (McClung,  '02,  p.  78).  McClung  nevertheless  maintained  the  exist- 
ence of  a  selective  power  on  the  part  of  the  egg  such  that  "  the  condition  of  the  ovum  determines 
which  sort  of  spermatozoon  shall  be  allowed  entrance  into  the  egg  substance"  (op.  cit.,  p.  76). 


Studies  on  Chromosomes  27 

the  spermatozoa  exists  in  Nezara,  yet  this  condition  is  con- 
nected, by  an  almost  continuous  series  of  intermediate  forms, 
with  one  in  which  a  conspicuous  difference  of  nuclear  con- 
stitution is  to  be  seen.  It  seems  hardly  open  to  doubt  that 
sex-production  conforms  to  the  same  essential  type  throughout 
this  series.  At  least  a  possibility  is  thus  established  that  in 
organisms  generally  both  eggs  and  spermatozoa  may  be  pre- 
destined as  male-producing  and  female-producing  forms,  whether 
they  are  visibly  different  or  not.  In  any  case,  it  is  evident 
that  in  the  Hemiptera  the  chromosome-combination  characteristic 
of  each  sex  is  established  by  union  of  the  gametes  and  is  a  result 
of  fertilization  by  one  or  the  other  of  the  two  forms  of  spermatozoa. 
Sex  must  therefore  already  be  predetermined  in  the  fertilized  egg, 
and  it  is  difficult  to  conceive  how  it  could  subsequently  be  altered 
in  these  animals  by  conditions  external  to  the  egg  or  embryo. 
Since  the  idiochromosomes  or  heterotropic  chromosomes  form  the 
distinctive  differential  between  the  nuclei  of  the  two  sexes,  it  is 
obvious  that  these  chromosomes  are  definitely  coordinated  with 
the  sexual  characters.  We  must  therefore  critically  inquire  into 
the  causal  relation  between  sex-production  and  the  chromosomes, 
of  which  this  coordination  is  an  expression. 

That  sex-production  may  be  interpreted  as  the  result  of  a 
Mendelian  segregation,  transmission  and  dominance  of  the  sexual 
characters  has  been  shown  by  Castle  ('03).  The  history  of  the 
differential  chromosomes  in  synapsis  and  reduction  evidently 
affords  a  concrete  basis  for  such  an  interpretation  in  the  terms  of 
the  Sutton-Boveri  chromosome-theory.  Analysis  of  the  facts  now 
known  will,  however,  show  even  more  clearly  than  the  more 
general  considerations  adduced  by  Castle,  that  this  interpretation 
is  only  admissible  under  the  assumption  that  a  selective  fertiliza- 
tion occurs,  such  that  eggs  containing  the  female-determinant  are 
fertilized  only  by  spermatozoa  containing  the  male-determinant 
and  vice  versa.  Until  I  had  read  Cuenot's  recent  interesting 
paper  ('05)  on  the  breeds  of  mice  and  their  combinations,  the 
necessity  for  making  this  assumption  seemed  to  me  an  almost 
fatal  difficulty  in  the  way  of  the  interpretation,  but  if  Cuenot's 
conclusions  be  well  founded  the  a  priori  objections  to  such  a 


28  Edmund  B.  Wilson 

selective  fertilization  are  in  large  measure  set  aside.  I  therefore 
think  that  the  possibility  of  a  Mendelian  interpretation  of  sex- 
production  should  be  carefully  examined,  though  as  will  be  shown, 
an  alternative  interpretation  is  possible. 

I.  In  such  an  examination  the  distinction  between  sex-deter- 
mination and  sex-inheritance  should  be  clearly  drawn;1  for  it  is 
well  known  that  each  sex  may  contain  factors  capable  of  pro- 
ducing the  characters  of  the  opposite  sex,  and  it  may  well  be  that 
the  patency  or  latency  of  the  sexual  characters  is  determined  by 
factors  quite  distinct  from  those  concerned  with  their  transmission 
from  parent  to  offspring.  For  the  purpose  of  analysis  it  will, 
however,  be  convenient  to  speak  of  the  idiochromosomes  or  their 
homologues  as  "sex-determinants,"  this  term  being  understood 
to  mean  that  these  chromosomes  are  the  bearers  of  the  male  and 
female  qualities  (or  the  factors  essential  to  the  production  of  these 
qualities)  respectively.  They  may  also  be  designated  (whenever 
it  is  desirable  to  avoid  circumlocution)  as  .sex-chromosomes  or 
"gonochromosomes."  As  a  basis  of  discussion  the  Mendelian 
interpretation  may  be  taken  to  postulate,  further,  that  the  two 
sex-chromosomes,  which  couple  in  synapsis  and  are  subsequently 
disjoined  by  the  reducing  division,  are  respectively  male-determi- 
nants and  female-determinants  in  the  sense  just  indicated.  The 
most  convenient  approach  to  the  question  is  offered  by  the  hetero- 
tropic  chromosome,  since  its  unpaired  condition  in  one  sex  renders 
its  mode  of  transmission  more  clearly  obvious  than  that  of  the 
idiochromosomes.  The  facts  (especially  as  observed  in  Protenor) 
clearly  prove  that  this  chromosome  alternates  between  the  sexes 
in  successive  generations,  passing  from  the  male  to  the  female  in 
the  production  of  females,  and  from  the  female  to  the  male  in  the 
production  of  males  (Fig.  6).  The  important  bearing  of  this 
on  both  sex-inheritance  and  sex-determination  will  appear  beyond. 

Since  the  heterotropic  chromosome  is  without  a  fellow  in  the 
male  it  must,  if  it  be  a  sex-determinant  at  all,  be  the  male-determi- 
nant, which  exerts  its  effect  uninfluenced  by  association  with  a 
female-determinant.  But  since  the  spermatozoa  that  contain 

»C/.  Watase,  '92. 


Studies  on  Chromosomes  29 

this  chromosome  produce  only  females,  it  must  be  assumed  that 
the  maternal  mate  or  fellow,  with  which  it  becomes  associated  on 
entering  the  egg,  is  a  dominant  female-determinant.  Further, 
since  males  result  from  fertilization  by  spermatozoa  that  do  not 
contain  the  heterotropic  chromosome,  the  latter  must  in  male- 
producing  eggs  be  derived  from  the  egg-nucleus  (cf.  the  diagram, 
Fig.  6).  The  general  interpretation,  therefore,  must  include  the 
assumption  that  there  are  two  kinds  of  eggs  (presumably  in 
approximately  equal  numbers)  that  contain  respectively  the  male- 
and  the  female-determinant,1  and  that  the  former  are  fertilized  only 
by  spermatozoa  that  lack  the  heterotropic  chromosome  (/.  e.,  the 
male  determinant)  and  vice  versa,2  giving  the  combinations  (m)f 
(female)  and  m — (male).  Such  a  selective  fertilization  is  there- 
fore a  sine  qua  non  of  the  assumption  that  the  heterotropic  chro- 
mosome is  a  specific  sex-determinant. 

A  nearly  similar,  though  somewhat  more  complex,  result  follows 
in  the  case  of  the  idiochromosomes.  In  respect  to  sex-production 
the  large  idiochromosome  is  identical  with  the  heterotropic  chro- 
mosome, and  the  morphological  evidence  is  nearly  or  quite 
decisive  that  the  heterotropic  chromosome  is  actually  a  large 
idiochromosome,  the  smaller  mate  of  which  has  disappeared. 
The  small  idiochromosome  may  therefore  be  regarded  as  a 
disappearing,  or  even  vestigial,  female-determinant  that  is  recessive 
to  its  larger  fellow  (the  male-determinant);  and  its  reduction  in 
size  may  plausibly  be  regarded  as  an  atrophy  resulting  from  its 
invariably  recessive  nature  (this  chromosome  being  strictly  con- 
fined to  the  male).  Precisely  as  in  case  of  the  heterotropic 
chromosome,  the  large  idiochromosome  of  the  male  (male- 
determinant)  must  be  derived  in  fertilization  from  the  egg-nucleus 
(Fig.  6);  and,  as  before,  it  must  be  assumed  that  eggs  that  contain 
this  chromosome  are  fertilized  only  by  spermatozoa  that  contain 
the  small  idiochromosome,  those  that  contain  the  female-determi- 

'This  would  follow  from  the  coupling  of  the  two  sex-chromosomes  in  synapsis  to  form  the  bivalent 
(m)j,  and  its  division  in  such  a  way  as  to  leave  in  the  egg  either  the  male-  or  the  female-determinant 
indifferently. 

Otherwise  the  combinations  mm  or  /—  might  result,  which  is  contrary  to  observation,  since  the  sex- 
chromosomes  are  in  this  type  never  paired  in  the  male  or  unpaired  in  the  female. 


30  Edmund  B.  Wilson 

nant  only  by  spermatozoa  containing  the  large  idiochromosome. 
In  this  type,  accordingly,  it  is  clear  that  the  large  idiochromosome 
(like  the  heterotropic  chromosome  to  which  it  corresponds)  passes 
alternately  from  one  sex  to  the  other,  while  the  small  one  never 
enters  the  female;  and  this  would  remain  true  even  did  selective  fer- 
tilization not  occur  (Fig.  6).  The  same  interpretation  may  finally 
be  extended  to  Nezara,  where  the  idiochromosomes  are  of  equal  size 
in  both  sexes,  the  relations  of  dominance  being  the  same  as  before. 
The  two  vital  points  in  this  result  are  first,  the  assumption  of 
selective  fertilization,  and  second  the  relations  of  dominance  and 
recession  in  the  two  sexes.  As  regards  the  first  point,  until  the 
appearance  of  Cuenot's  paper,  referred  to  above,  almost  no 
definite  evidence  had  been  produced  of  an  infertility  between 
particular  classes  of  gametes  in  the  same  species;  though  it  has 
long  been  known  that  many  plants  are  in  a  greater  or  less  degree 
infertile  to  their  own  pollen,  and  an  analogous  fact  has  been  more 
recently  demonstrated  in  Ciona  by  Castle  ('96)  and  Morgan  ('04). 
Correns  ('02),  in  his  study  of  hybrid  maize,  was  led  to  suggest 
that  in  this  case  there  might  be  a  somewhat  diminished  fertility 
between  the  gametes  bearing  the  recessive  character  (thus  account- 
ing for  a  relative  deficiency  of  extracted  recessives  in  the  second 
generation  of  crosses,  F2).  In  studying  the  breeds  of  mice 
Cuenot  has  found  it  impossible  to  obtain  pure  or  homozygous  yellow 
forms.  Yellow  mice  are  invariably  heterozygotes  (the  yellow 
being  dominant  over  gray,  black  or  brown)  and  when  crossed 
with  a  pure  race  of  a  different  color  (e.  g.,  gray)  give  the  typical 
Mendelian  result,  yellow  and  gray  offspring  appearing  in  equal 
numbers.  This  proves  that  a  complete  Mendelian  disjunction 
of  the  yellow  and  gray  determinants  takes  place  in  maturation. 
When  yellow  mice  of  known  constitution  (e.  g.,  Y(G))  are  paired 
with  like  forms,  the  first  offspring  include  pure  gray  forms  (ex- 
tracted recessives)  slightly  in  excess  of  the  normal  ratio  of  25  per 
cent.,  and  yellow  forms;  but  contrary  to  the  Mendelian  expec- 
tation the  latter,  when  paired  with  one  another,  never  give  pure 
dominants  (YY),  but  again  produce  pure  grays  (GG)  and 
heterozygous  yellows  (Y(G)).  Cuenot  therefore  concludes  that 
although  complete  segregation  of  both  the  gray  and  yellow 


Studies  on  Chromosomes  31 

characters  takes  place  in  the  gamete-formation,  and  the  resulting 
yellow-bearing  gametes  unite  freely  with  those  bearing  the  reces- 
sive color,  they  do  not  unite  with  each  other:  "Ceux-ci  (the 
yellow  heterozygotes)  forment  bien  des  gametes  de  valeur  CJ  ou 
AJ,  mais  ces  gametes  ne  peuvent  pas  s'unir  les  uns  aux  autres  pour 
donner  des  zygotes  ayant  les  formules  CJCJ,  AJAJ  ou  CJAJ; 
par  autre,  ils  s'unissent  facilement  a  tous  les  autres  gametes  que 
j'ai  essay es  pour  former  avec  eux  des  heterozygotes  mono-  ou 
dihybrides"  (op.  cit.,  p.  cxxx).  This  conclusion  is  sustained  by 
the  fact  that  the  combination  Y(G)  x  Y(G)  (CYCG  x  CYCG  in 
Cuenot's  terminology)  produces  a  relative  deficiency  of  yellows 
in  the  offspring,  as  is  to  be  expected.1  In  pairing  Y(G)  with 
Y(G),  accordingly,  the  Y-bearing  spermatozoa  unite  only  with 
the  G-bearing  eggs,  and  vice  versa,  which  is  exactly  analogous 
to  the  selective  fertilization  assumed  in  case  of  the  sex-bearing 
gametes.  Perhaps  it  may  be  possible  to  find  a  different  expla- 
nation of  the  facts;  but  if  Cuenot's  interpretation  be  well-founded 
the  case  goes  far  to  remove  the  scepticism  which  I  think  one  must 
otherwise  feel  in  regard  to  a  selective  fertilization  of  the  gametes 
in  sex-production. 

An  examination  of  the  question  of  dominance  involved  in  the 
Mendelian  interpretation  leads  to  some  interesting  conclusions. 
In  forms  possessing  unequal  idiochromosomes  the  sexual  formulas 
would  be  for  the  female  (m)  f  and  for  the  male  m  (/)  (/  being  the 
small  idiochromosome).  Applying  the  same  interpretation  to 
Nezara,  where  the  idiochromosomes  are  of  equal  size,  the  corre- 
sponding formulas  are  (m)  f  and  m  (/),  giving  the  gametes  (m), 
/,  m  and  (/).  Assuming  likewise  a  selective  fertilization  the  facts 
would  be: 

EGGS        SPERMATOZOA 

(«)      +       (/)  =  (m)  (/)»  producing  a  male,  m(f). 

f       +        m  =    mf,  producing  a  female  (m)  /. 

rfhe  deficiency,  though  constant,  is  very  slight.  Cuenot  himself  seems  to  consider  this  a  difficulty, 
but  I  believe  a  very  simple  explanation  may  be  given.  With  equal  numbers  of  the  gametes  of  both  sexes 
the  ratio  of  yellows  to  grays  should  be  two  to  one,  instead  of  three  to  one  as  in  the  typical  Mendelian  case 
(since  the  class  YY  is  missing).  If,  however,  the  spermatozoa  be  in  large  excess,  as  they  undoubtedly 
are,  all  or  nearly  all  the  Y-bearing  eggs  will  be  fertilized  by  G-bearing  spermatozoa,  and  vice  vena,  thus 
bringing  the  ratio  of  yellows  (Y(G))  to  grays  (GG)  more  or  less  nearly  up  to  three  to  one. 


32  Edmund  B.  Wilson 

Now  it  is  clear  that  if  the  relations  of  the  chromosomes  to  sex- 
production  be  the  same  here  as  in  the  second  type,  the  chromo- 
some m  must  alternate  in  successive  generations  between  the  male 
and  the  female  (like  the  large  idiochromosome  or  the  heterotrqpic 
chromosome  to  which  it  corresponds),  and  hence  also  shows  an 
alternation  of  dominance,  being  dominant  in  the  former  sex  and 
recessive  in  the  latter.  If,  therefore,  dominance  and  recession 
be  inherent  in  the  chromosomes,  there  must  be  such  a  relation 
between  them  that  m  is  always  dominant  to  the  chromosome  (/) 
of  the  male,  and  always  recessive  to  the  chromosome  /  of  the 
female,  and  that  the  latter  two  chromosomes  (/  and  (/))  are  never 
interchanged  between  the  sexes.  This  last  assumption  is  not  so 
improbable  as  it  may  at  first  sight  appear;  for  in  the  second  type 
it  is  certain,  as  already  pointed  out,  that  the  small  idiochromo- 
some ((/)  under  the  general  assumption)  never  enters  the  female, 
while  the  large  idiochromosome,  m,  like  the  heterotropic,  alter- 
nates between  the  two  sexes  in  successive  generations. 

A  strict  Mendelian  interpretation  of  sex-production  may  unques- 
tionably, I  think,  be  constructed  upon  the  foregoing  assumptions. 
But  an  interesting  suggestion  for  a  somewhat  modified  Mendelian 
interpretation  is  given  by  the  possibility  that  the  dominance  of 
the  sex-chromosomes  is  determined  by  extrinsic  factors,  namely, 
by  conditions  in  the  protoplasm  of  the  zygote.  If  this  were  the 
case  it  is  evident  that  the  idiochromosomes  could  not  be  considered 
as  sex-determinants  in  the  strict  sense  of  the  word.  The  determi- 
nation of  sex  would  in  this  case  be  due  to  factors  preexisting  in 
one  or  both  of  the  gametes,  irrespective  of  the  sex-chromosomes, 
and  the  latter  could  only  be  considered  as  a  means  by  which  the 
sex-characters  are  transmitted  or  inherited.  The  possibility  is 
here  clearly  offered  that  either  or  both  forms  of  gametes  may  be 
predetermined  as  males  or  females  (or  at  least  male-producing 
and  female-producing)  prior  to  fertilization  and  irrespective  of  the 
chromosomes;  and  thus  an  interpretation  of  the  ordinary  forms 
of  gametes  would  be  reached  in  harmony  with  such  cases  as 
Dinophilus  and  other  forms  in  which  male-producing  and  female- 
producing  eggs  are  distinguishable  in  size  prior  to  fertilization. 
Such  an  interpretation  would  further  be  perfectly  consistent  with 


Studies  on  Chromosomes  33 

the  modification  of  sex-production  in  some  cases  by  external  condi- 
tions, and  with  the  production  of  both  males  and  females  in 
parthenogenesis  (though  this  may  be  otherwise  explicable);  and 
it  might  also  give  the  explanation  of  selective  fertilization. 

II.  It  has  not  been  my  intention  to  advocate  the  foregoing 
interpretation,  but  only  to  set  forth  as  clearly  as  possible,  the  as- 
sumptions that  it  involves.  It  is  nevertheless  my  opinion  that  the 
analysis  places  no  insuperable  obstacles  in  its  way,  and  that, 
however  dominance  be  determined,  the  Mendelian  interpretation 
may  in  fact  give  the  true  solution  of  the  problem.  I  have,  how- 
ever, endeavored  to  seek  for  a  different  interpretation  that  may 
escape  the  necessity  for  assuming  a  selective  fertilization;  and 
although  I  have  to  offer  nothing  more  than  suggestions,  some  of 
which  undoubtedly  encounter  serious  difficulties,  I  shall  make 
them  in  the  hope  that  they  may  afford  some  clue  to  further  inquiry. 
Some  of  these  suggestions  are  equally  applicable  to  the  Mendelian 
interpretation  considered  above,  but  for  the  purpose  of  discussion 
this  interpretation  may  for  the  time  be  laid  aside. 

It  seems  possible  that  the  differential  chromosomes  may  per- 
form a  definite  and  special  function  in  sex-production  without  being 
in  themselves  specifically  male-determining  and  female-determin- 
ing or  even  qualitatively  different  save  in  the  degree  of  their  special 
activity  (whatever  be  its  nature).  This  suggestion  is  given  by 
the  fact  that  the  presence  of  one  heterotropic  chromosome  or 
large  idiochromosome  is  associated  with  the  production  of  a 
male,  while  if  two  such  chromosomes  are  present  a  female  is 
produced.  This  very  obviously  suggests  that  the  same  kind  of 
activity  that  produces  a  male  will  if  reinforced  or  intensified 
produce  a  female;  and  with  this  would  accord  the  production 
of  males  from  unfertilized  eggs,  and  females  from  fertilized  ones, 
in  the  case  of  the  bee.  In  these  cases  the  decisive  factor  may  be 
a  merely  quantitative  difference  of  chromatin  between  the  two 
sexes.  But  it  is  obvious  that  such  a  difference  cannot  give  the 
basis  for  a  general  explanation,  since  in  Nezara,  and  presumably 
in  many  other  organisms,  both  the  number  of  chromosomes  and 
the  quantity  of  chromatin  is  the  same  in  both  sexes.  And  yet 


34  Edmund  B.  Wilson 

the  existence  of  a  quantitative  difference  in  some  cases  raises  the 
question  whether  it  is  not  the  result  or  expression  of  some  more 
deeply  lying  nuclear  difference  which  may  still  be  present  in  those 
cases  where  no  quantitative  difference  exists.  I  find  it  altogether 
incredible  that  two  animals  as  nearly  related  as  Nezara  and 
Euschistus  should  differ  fundamentally  in  the  relation  of  the 
chromosomes  to  sex-production;  and  if  there  is  any  reason  to 
conclude  that  sex-determination  is  effected  by  the  idiochromo- 
somes  (or  by  the  combination  of  which  they  form  a  part)  in  the 
case  where  they  are  visibly  different,  I  cannot  avoid  the  belief 
that  this  conclusion  applies  with  equal  reason  to  the  case  in  which 
they  appear  to  the  eye  alike  in  all  the  spermatozoa.  It  therefore 
seems  to  me  an  admissible  hypothesis  that  a  physiological  or 
functional  factor  may  be  present  that  differentiates  the  spermat- 
ozoa into  male-producing  and  female-producing  forms  irrespec- 
tive of  the  size  of  the  differential  chromosomes;  and  further, 
that  the  morphological  difference  that  has  arisen  in  some  forms 
may  have  been  a  consequence  of  such  an  antecedent  functional 
difference.  If  we  could  assume  for  instance  that  the  differential 
chromosome-pair  in  the  male  includes  a  more  active  and  a  less 
active  member  (the  latter  having  in  many  cases  become  reduced 
in  size  or  even  having  entirely  disappeared)  the  suggestion  might 
be  greatly  extended  in  application.  Under  this  assumption  the 
facts  might  receive  a  general  formulation  in  the  statement  that 
the  association  of  two  more  active  chromosomes  of  this  class 
produces  a  female,  while  the  association  of  a  more  active  and  a 
less  active  one  (or  the  absence  of  the  latter,  as  in  case  of  the  hetero- 
tropic  chromosome)  produces  a  male.  Reduction  of  the  less 
active  member  to  form  a  small  idiochromosome  would  introduce 
a  quantitative  difference  of  chromatin  as  well  as  a  qualitative  one. 
Its  complete  disappearance  in  the  male,  leaving  only  the  active 
member  as  the  heterotropic  chromosome,  would  reduce  the 
difference  to  a  merely  quantitative  one.  The  assumption  of 
such  a  physiological  difference  is  admittedly  a  purely  specula- 
tive construction,  and  may  seem  a  priori  very  improbable.  But 
from  the  a  priori  point  of  view  it  would  seem  equally  improbable 
that  a  morphological  dimorphism  of  the  spermatozoa,  affecting 


Studies  on  Chromosomes  35 

only  one  pair  of  the  chromosomes,  should  have  arisen;  yet  this 
is  an  observed  fact.  I  therefore  think  the  suggestion  is  worthy 
of  serious  consideration.  If  it  could  be  adopted  the  necessity 
of  selective  fertilization  would  be  avoided,  for  the  observed  results 
would  follow  from  the  fertilization  of  any  egg  by  any  spermatozoon. 

But  even  if  in  accordance  with  fact  the  suggestion  is  still 
obviously  incapable  of  direct  application  to  cases  in  which  sex 
is  determined  independently  of  fertilization — for  instance,  sex- 
production  in  parthenogenetic  development  or  in  hermaphrodites, 
and  in  forms  (such  as  Dinophilus)  where  male-producing  and 
female-producing  eggs  are  distinguishable  in  size  before  fertiliza- 
tion. It  is  possible  that  these  cases  may  be  explicable  (under  either 
general  interpretation)  as  a  result  of  some  forms  of  differential 
distribution  of  the  chromosomes  occurring  at  the  time  of  the  for- 
mation of  the  polar  bodies  (parthenogenesis)  or  at  some  earlier 
period  in  the  cell-lineage  of  the  germ-cells;  and  this  possibility 
should  of  course  be  tested  by  a  close  cytological  study  of  the  facts. 
On  the  other  hand,  there  is  nothing  in  the  facts  to  negative  the 
assumption  that  in  some  cases  the  chromosome-combination, 
established  at  fertilization,  may  be  in  something  like  a  balanced 
state  that  is  capable  of  modification  by  conditions  external  to  the 
nucleus  (as  already  suggested  in  the  case  of  dominance). 

Boveri's  interesting  observations  on  the  dispermic  eggs  of 
Ascaris  ('04)  have  given  direct  evidence  that  the  chromosomes 
react  to  their  cytoplasmic  surroundings;  and  the  same  fact  is 
even  more  clearly  shown  by  the  difference  of  behavior  of  the 
differential  chromosomes  in  the  two  sexes  of  Hemiptera  during  the 
synaptic  and  growth-periods.  Hence,  even  though  a  preestab- 
lished  basis  of  sex-determination  be  given  in  such  a  physiological 
dimorphism  of  the  spermatozoa  as  I  have  suggested,  the  sex  of 
the  fertilized  eggs  may  in  many  cases  be  only  a  matter  of  greater 
or  less  predisposition  and  not  an  immutable  predetermination. 
The  nuclei,  and  hence  the  primordial  germ-cells,  may  in  such 
cases  be  in  a  state  of  approximate  equilibrium,  and  still  retain  the 
power  of  response  to  varying  conditions  in  the  cellular  environ- 
ment. The  production  of  eggs  or  spermatozoa  in  hermaphro- 
dites may  thus  be  explicable  as  a  result  of  greater  or  less  nuclear 


36  Edmund  B,  Wilson 

activity  in  the  two  cases,  incited  by  intra-cellular  conditions  that 
are  external  to  the  chromosome-groups;  and  a  similar  explana- 
tion may  apply  to  the  related  case  of  the  formation  of  visibly 
different  female-producing  and  male-producing  eggs  in  the  same 
organism. 

It  would  not,  I  think,  be  profitable  to  speculate  further  in  regard 
to  these  special  cases,  but  I  have  wished  to  indicate  that  a  hypo- 
thesis of  sex-production  which  recognizes  in  some  cases  a  fixed 
predetermination  in  the  chromosome-groups  of  the  fertilized  egg 
is  not  inconsistent  with  the  control  of  sex-production  in  other 
cases  by  conditions  external  to  the  nucleus.  The  constant 
chromosomal  differences  of  the  sexes  existing  in  many  Hemiptera, 
therefore,  by  no  means  preclude  experiments  on  the  modification 
or  control  of  sex-production. 

I  have  intentionally  excluded  from  the  foregoing  suggestions 
any  discussion  of  the  specific  nature  of  the  activities  of  the  differen- 
tial chromosomes,  since  we  are  almost  wholly  ignorant  of  the 
functions  of  chromosomes  in  general.  But  although  we  here 
enter  upon  still  more  debatable  ground,  I  think  we  should  not 
hesitate  to  consider  such  possibilities  in  this  direction  as  the  facts 
may  suggest. 

One  of  the  principal,  or  at  least  most  obvious,  differences 
between  the  germ-cells  of  the  two  sexes  is  their  great  contrast  in 
constructive  activity,  evinced  by  the  enormous  growth  of  the 
primary  oocyte  as  compared  with  the  primary  spermatocyte. 
This  growth  of  the  oocyte  involves  the  production  of  a  mass  of 
protoplasm  (including  under  this  term  the  yolk  or  metaplasm  as 
well  as  the  active  protoplasm)  thousands  of  times  the  bulk  of  the 
spermatocyte;  and  although  the  latter  also  increases  noticeably 
in  size  during  the  growth-period,  the  accumulation  of  proto- 
plasm is  almost  insignificant  as  compared  with  that  which  takes 
place  in  the  female.  Now,  as  described  above,  the  idiochromo- 
somes  and  heterotropic  chromosome  remain  during  this  period  in 
the  male  in  a  relatively  passive  condition  as  compared  with  the 
other  chromosomes,  while  this  is  not  the  case  in  the  female.  The 
thought  cannot  be  avoided  that  there  is  a  definite  causal  connec- 

o 

tion  between  the  greater  activity  of  these  chromosomes  in  the 


Studies  on  Chromosomes  37 

oocytes  and  the  great  preponderance  of  constructive  activity  in 
these  cells;  and  it  is  especially  this  coincidence  that  leads  me  to 
the  general  surmise  that  one  of  the  important  physiological 
differences  (I  do  not  say  the  only  one),  between  the  chromosome- 
groups  of  the  two  sexes,  may  be  one  of  constructive  activity. 
I  have  elsewhere  (The  Cell,  Chapter  VII)  reviewed  at  some 
length  the  evidence  pointing  toward  the  conclusion  that  the 
nucleus  (more  specifically,  the  chromatin)  is  especially  concerned 
with  the  constructive  processes  of  cell  metabolism;  and  while  I  no 
longer  hold  the  view  that  the  nucleus  can  be  considered  as  the 
actual  formative  center  of  the  cell,  it  still  seems  to  me  very 
probable  that  the  formative  processes  are  directly  or  indirectly 
under  its  control,  as  has  been  advocated  by  many  students  of 
cell-physiology.  If  this  view  be  well-founded,  the  facts  observed 
in  Hemiptera  give  a  very  definite  and  concrete  basis  for  assuming 
a  greater  constructive  activity  in  the  cells  of  the  female  generally, 
which  reaches  a  climax  in  the  growth-period  of  the  oocyte.1  It 
seems  possible  that  some  of  the  specific  differentiations  that  take 
place  in  the  later  history  of  the  germ-cells  may  be  directly  trace- 
able to  the  primary  difference  in  the  growth-process.  It  is  well 
known  that  the  young  oocytes  and  spermatocytes  show  a  very 
close  similarity,  not  only  in  size  but  also  in  many  details  of  struc- 
ture. The  enormous  accumulation  of  cytoplasm  in  the  oocyte 
as  compared  with  the  spermatocyte  leaves  the  latter  with  a  great 
relative  excess  of  the  kinoplasmic  or  archoplasmic  material  in  which 
the  most  characteristic  differentiations  of  the  spermatozoa — such  as 
the  acrosome,  middle-piece,  axial  filament  and  tail-envelopes— 
take  their  origin.  Perhaps  a  direct  causal  relation  here  exists. 

'This  suggestion  recalls  the  theory  developed  by  Geddes  and  Thomson,  in  their  well  known  work  on 
the  "Evolution  of  Sex,"  that "  the  female  is  the  outcome  and  expression  of  relatively  preponderant  anabo- 
lism,  and  the  male  of  relatively  preponderant  katabolism"  (pp.  cit.,  revised  ed.,  1901,  p.  140).  As  de- 
deloped  by  these  authors,  this  theory  has  always  seemed  to  me  to  have  too  vague  and  general  a  character 
to  have  much  practical  value,  though  it  expresses  a  certain  physiological  contrast  between  the  sexes  that 
undoubtedly  exists.  My  suggestion  is  only  remotely  connected  with  that  theory,  since  it  refers  the  differ- 
entiation of  the  sexes  to  a  functional  difference  that  preexists  in  the  cells  of  the  male,  and  involves  no 
contrasted  processes  of  anabolism  and  katabolism.  Nevertheless,  the  observations  here  brought  forward 
may  harmonize  with  that  side  of  the  theory  which  lays  stress  on  the  preponderant  constructive  activity  of 
the  female  cells. 


38  Edmund  B.  Wilson 

III.  Though  I  have  found  it  convenient  to  consider  the  two 
foregoing  interpretations  separately,  they  evidently  have  many 
points  of  agreement,  and  perhaps  may  be  reduced  to  a  common 
basis.  Both  assign  to  the  differential  chromosomes  a  specific 
function  in  sex-production,  both  recognize  the  possibility  of  a 
determination  of  sex  (as  opposed  to  its  transmission),  by  con- 
ditions external  to  the  chromosome-groups,  and  both  assume, 
in  one  sex,  a  specific  difference  in  the  sex-chromosomes,  followed 
by  a  Mendelian  disjunction  in  the  formation  of  the  gametes. 
The  essential  point  in  which  the  second  interpretation  diverges 
from  the  first  is  that  the  sex-chromosomes  are  not  conceived  as 
bearing  the  male  or  female  qualities  respectively  but  as  differing 
only  in  the  degree  of  their  activity,  and  this  difference  is  assumed 
to  exist  in  the  male  only  (owing  to  the  relation  of  fertilization 
to  sex-production).  It  must  be  admitted  that  each  interpreta- 
tion involves  a  considerable  element  of  pure  conjecture,  and  that 
each  includes  assumptions  which  without  additional  data  must 
be  considered  as  serious  difficulties.  The  principal  one  involved 
in  the  first  interpretation  is  the  assumption  of  selective  fertiliza- 
tion; but  if  this  assumption  be  granted  I  believe  that  it  may  give 
an  adequate  solution  of  the  problem  of  sex-production  in  the  sexual 
reproduction  of  dioecious  organisms.  The  second  interpreta- 
tion avoids  this  difficulty;  it  may  explain  the  primary  difference 
between  the  gametes  of  the  two  sexes,  the  latency  of  female 
characters  in  the  male,  and  the  development  of  such  secondary 
female  characters  as  may  be  regarded  as  an  exaggeration  or  inten- 
sification of  corresponding  characters  in  the  male.  It  seems  con- 
spicuously to  fail  to  explain  the  reverse  case  of  characters  that 
are  more  highly  developed  in  the  male;  and  to  many  this  will 
doubtless  appear  a  fatal  difficulty.  But  we  are  still  ignorant  of 
the  action  and  reaction  of  the  chromosomes  on  the  cytoplasm  and 
on  one  another,  and  have  but  a  vague  speculative  notion  of  the 
relations  that  determine  patency  and  latency  in  development. 
Additional  data  will  therefore  be  required,  I  think,  to  show 
whether  the  difficulty  in  question  is  a  fatal  one,  and  in  what  meas- 
ure either  of  the  two  general  interpretations  that  have  been  con- 
sidered may  approach  the  truth.  The  positive  result  of  the 


Studies  on  Chromosomes  39 

observations  of  Stevens  and  myself  is  to  demonstrate  the  existence 
of  a  constant  and  definite  correlation  between  the  chromosomes 
and  the  sexual  characters,  which  is  visibly  expressed  in  the  relations 
of  a  single  pair  of  chromosomes.  These  relations  unquestionably 
afford  a  concrete  basis  for  an  interpretation  of  sex-production 
that  assumes  a  Mendelian  segregation  and  transmission  of  the 
sex-characters  and  to  this  extent  they  accord  with  the  general 
assumption  of  Castle.  The  validity  of  both  this  and  the  alterna- 
tive interpretation  suggested  must  be  tested  by  further  inquiry. 

Zoological  Laboratory  of  Columbia  University, 
December  8,  1905. 

WORKS   CITED 

BEARD,  JOHN,  '02. — The  Determination  of  Sex  in  Animal  Development.     Fischer, 

Jena. 
BOVERI,  T.  H.,  '04. — Protoplasmadifferenzierung  als  auslosender  Faktor  fur  Kern- 

verschiedenheit.     Sitzungsber.    der    Physikal.-med.    Ges.    Wiirz- 

burg,  1904. 
CASTLE,  W.  E.,  '96. — The  Early  Embryology  of  Ciona  intestinalis.     Bull.  Mus. 

Comp.  Zool.,  xxvii. 

'03. — The  Heredity  of  Sex.     Ibid.,  xl,  4. 
CORRENS,  C.,  '02. — Scheinbare  Ausnahme  von  der  Mendel'schen  Spaltungsregel 

fur  Bastarde.     Ber.  d.  deutschen  Bot.  Ges.,  xx. 
CUENOT,  L.,  '99. — Sur  la  determination  du  sexe  chez  les  animaux.     Bull.  Sci.  de 

la  France  et  de  la  Belgique,  xxxii,  v,  I. 
'05. — Les  races  pures  et  leurs  combinaisons  chez  les  souris.     Arch.  Zool. 

Exp.  et  Gen.  (4),  iii,  Notes  et  Revue,  No.  7. 
GROSS,  J.,  '04. — Die  Spermatogenese  von  Syromastes  marginatus.     Zool.  Jahrb., 

Anat.  und  Ontog.,  xx,  3. 
HENKING,  H.,  '91. — Ueber  Spermatogenese  und  deren  Beziehung  zur  Eientwick- 

lung  bei  Pyrrochoris  apterus.     Zeitschr.  Wiss.  Zool.,  li. 
LENHOSSEK,  M.  v.,  '03. — Das  Problem  der  geschlechtsbestimmenden  Ursachen. 

Fischer,  Jena. 
McCLUNG,  C.  E.,  '02. — The  Accessory  Chromosome.     Sex-determinant?     Biol. 

Bull.,  iii,  I,  2. 
MONTGOMERY,  T.  H.,  '01. — A  Study  of  the  Germ  Cells  of  Metazoa.     Trans. 

Am.  Phil.  Soc.,  xx. 

'04. — Some  Observations  and  Considerations  on  the  Maturation  Phenom- 
ena of  the  Germ-cells.     Biol.  Bull.,  vi,  3. 


40  Edmund  B.  Wilson 

MORGAN,  T.  H.,  '04. — Self-fertilization  Induced  by  Artificial  Means.  Jour. 
Exp.  Zool.,  i,  I. 

SCHULTZE,  O.,  '03. — Zur  Frage  von  den  geschlechtsbildenden  Ursachen.  Arch, 
mik.  Anat.,  Ixiii. 

STEVENS,  N.  M.,  '05. — Studies  in  Spermatogenesis  with  especial  Reference  to  the 
"Accessory  Chromosome."  Publication  No.  36,  Carnegie  Insti- 
tution of  Washington,  Sept.,  1905. 

STRASBURGER,  E.,  1900. — Versuche  mit  diocischen  Pflanzen  in  Riicksicht  auf 
Geschlechtsverteilung.  Biol.  Centralbl.,  xx,  20-24. 

SUTTON,  W.  S.,  '02. — On  the  Morphology  of  the  Chromosome-group  in  Brachy- 
stola  magna.  Biol.  Bull.,  iv,  I. 

WALLACE,  L.  B.,  '05. — The  Spermatogenesis  of  the  Spider.     Biol.  Bull.,  viii,  3. 

WATASE,  S.,  '92. — On  the  Phenomena  of  Sex-differentiation.  Journ.  of  Mor- 
phology, vi,  3. 

WILSON,  E.  B.,  '05,  I. — Studies  on  Chromosomes.     I.   The  Behavior  of  the  Idio- 

chromosomes  in  Hemiptera.     Journ.  Exp.  Zool.,  ii,  3. 
'05,  2. — The  Chromosomes  in  Relation  to  the  Determination  of  Sex  in 

Insects.     Science,  xxii,  564,  Oct.  20,  1905. 

'05,  2.  Studies  on  Chromosomes.  II.  The  Paired  Microchromosomes, 
Idiochromosomes  and  Heterotropic  Chromosomes  in  Hemiptera. 
Journ.  Exp.  Zool.,  ii,  4. 


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STUDIES  ON  CHROMOSOMES 

IV     THE  "ACCESSORY"  CHROMOSOME  IN  SYROMASTES  AND 

PYRROCHORIS  WITH  A  COMPARATIVE  REVIEW  OF 

THE  TYPES  OF  SEXUAL  DIFFERENCES  OF 

THE  CHROMOSOME  GROUPS 


By 
EDMUND  B.  WILSON 


RETURN  TO 

DIVISION  OF  GENETICS 

HILGARD  HALL 


REPRINTED     FROM 


THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY 


Volume  VI         No.  i 


JANUARY,   1909 


BALTIMORE,  MD.,  U.  S.  A. 
WILLIAMS  &  WILKINS  COMPANY 


STUDIES  ON  CHROMOSOMES 

IV  THE  "ACCESSORY"  CHROMOSOME  IN  SYROMASTES  AND 
PYRROCHORIS  WITH  A  COMPARATIVE  REVIEW  OF  THE 
TYPES  OF  SEXUAL  DIFFERENCES  OF  THE  CHROMOSOME 
GROUPS1 

BY 

EDMUND  B.  WILSON 
WITH  Two  PLATES  AND  Two  FIGURES  IN  THE  TEXT 

Since  the  unpaired  idiochromosome  ("accessory  chromosome") 
was  first  discovered  by  Henking  ('91)  in  Pyrrochoris  apterus  L.  this 
species  has  been  reexamined  by  only  one  observer,  Dr.  J.  Gross 
('07),  with  results  that  are  in  substantial  agreement  with  those  that 
pe  had  reached  in  an  earlier  investigation  ('04)  on  the  coreid 
species  Syromastes  marginatus  L.  In  both  cases  his  conclusions 
hre  in  conflict  with  the  view  advanced  by  McClung  ('02),  and  first 

1  Terminology.  With  the  advance  of  our  knowledge  of  the  chromosomes  that  form  the  distinctive 
differential  between  the  chromosome  groups  of  the  two  sexes,  and  between  the  male  producing  and  the 
female  producing  spermatozoa,  it  becomes  increasingly  difficult  to  find  a  common  name  that  will  apply 
equally  to  their  various  modifications.  Terms  such  as  the  "accessory,"  "odd"  or  "heterotropic"  chro- 
mosome, or  "monosome,"  that  are  based  on  the  condition  of  these  chromosomes  in  the  male  only,  are 
misleading  or  inappropriate;  and  some  of  them  are  in  certain  cases  inapplicable,  even  in  the  male — 
e.  g.,  in  Syromastes,  where  the  "accessory"  chromosome  is  not  univalent  but  bivalent.  Such  terms  as 
"heterochromosome"  or  "allosome"  (Montgomery)  seem  to  me  unsatisfactory,  since  they  designate  the 
m-chromosomes  as  well  as  the  differential  chromosomes,  though  these  are  obviously  of  quite  different 
nature.  Since  it  has  now  become  evident  that  a  univalent  "accessory"  chromosome  in  the  male  is 
exactly  equivalent  to  what  I  have  called  the  "large  idiochromosome"  in  other  forms,  I  think  these 
chromosomes  should  be  designated  by  the  same  name,  and  one  that  will  apply  equally  to  both  sexes. 
While  there  are  some  objections  to  the  word  "idiochromosome"  as  a  general  term  for  this  purpose  I 
am  not  able  to  suggest  a  better  one;  and  since  it  has  already  teen  thus  employed  by  some  writers,  I 
shall  use  it  hereafter  in  a  broader  sense  than  that  in  which  I  first  proposed  it,  to  designate  the  differential 
chromosomes  in  general,  whether  they  are  paired  or  unpaired  in  the  male,  and  whether  one  or  more 
pairs  are  present.  A  univalent  or  odd  idiochromosome  in  the  male  will  be  called  the  unpaired  idiochro- 
mosome (or  simply  the  idiochromosome),  while  the  word  "heterotropic"  may  perhaps  conveniently 
be  used  as  descriptive  of  its  passage  without  division  to  one  pole  in  one  of  the  maturation  divisions.  In 
Syromastes,  as  will  appear,  the  "accessory"  or  heterotropic  chromosome  represents  a  pair  of  idiochro- 
mosomes;  while  in  Galgulus  there  are  several  pairs  of  these  chromosomes. 
THE  JOURNAL  or  EXPERIMENTAL  ZOOLOGY,  VOL.  vi,  NO.  I. 


JO  Edmund  B.   Wilson 

shown  to  be  correct  in  principle  by  the  work  of  Stevens  and  my- 
self, that  half  the  spermatozoa  are  male  producing  and  half  female 
producing.  This  view  rests  on  the  following  facts.  When  the 
male  somatic  chromosome  groups  contain  an  odd  number,  includ- 
ing an  odd  or  unpaired  idiochromosome  (as  in  Anasa,  Alydus,  or 
Protenor)  the  female  groups  have  one  more  chromosome,  being 
duplicates  of  the  male  groups  with  the  addition  of  another  chro- 
mosome like  the  unpaired  one  of  the  male.  When  the  male  groups 
contain  an  even  number,  including  a  large  and  a  small  idiochro- 
mosome (as  in  Lygaeus,  Coenus  or  Tenebrio)  the  female  groups 
contain  the  same  number,  but  include  two  large  idiochromosomes 
in  place  of  a  large  and  a  small  one.  In  the  first  type  half  the 
spermatozoa  receive  the  odd  idiochromosome  while  half  do 
not,  the  former  accordingly  containing  one  chromosome  more 
than  the  latter.  In  the  second  type  all  the  spermatozoa  receive 
the  same  number  of  chromosomes,  but  half  receive  the  large 
idiochromosome  and  half  the  small.  It  follows  from  these  rela- 
tions that  eggs  fertilized  by  spermatozoa  containing  the  odd  chro- 
mosome, or  its  homologue  the  large  idiochromosome,  must  pro- 
duce females,  those  fertilized  by  the  other  spermatozoa  males. 
These  cytological  results,  first  reached  by  Stevens  ('05)  in  Tene- 
brio (which  has  a  pair  of  unequal  idiochromosomes  in  the  male) 
and  myself  ('o5b,  'o5c,  '06)  in  Anasa,  Protenor,  Alydus  and 
Harmostes  (which  have  an  unpaired  idiochromosome  in  the  male) 
and  in  Lygaeus,  Coenus,  Podisus  and  Euschistus  (which  agree 
essentially  with  Tenebrio),  have  since  been  confirmed  in  a  con- 
siderable number  of  species  and  extended  to  several  other  orders 
of  insects.2  They  have  recently  received  indirectly  a  striking 
experimental  confirmation  in  the  important  work  of  Correns  ('07), 
which  proves  that  in  the  dioecious  flowering  plant,  Bryonia  dioica, 
the  pollen  grains  are  likewise  male  determining  and  female  deter- 
mining in  equal  numbers. 

Gross's  conclusion  in  the  case  of  Syromastes  and  Pyrrochoris 
is  opposed  to  all  these  results  in  that  only  one  of  the  two  forms  of 
spermatozoa  is  supposed  to  be  functional  (those  containing  the 

a  See  the  tabular  review  in  the  sequel. 


Studies  on  Chromosomes  JI 

"accessory"  chromosome)  the  others  being  regarded  as  in  a  certain 
sense  comparable  to  polar  bodies  (as  was  also  supposed  by  Wallace 
('05). 3  This  result  was  based  mainly  on  the  numerical  relations, 
and  especially  on  the  belief  that  in  both  these  forms  the  number  of 
chromosomes  is  an  even  one  and  the  same  in  both  sexes — twenty- 
two  in  Syromastes,  twenty-four  in  Pyrrochoris.  Since  the  com- 
plete reduced  number  (eleven  and  twelve  in  the  two  respective 
cases)  is  present  only  in  those  spermatozoa  that  contain  the 
"accessory"  chromosome,  Gross  argues  that  this  class  alone  can 
be  concerned  in  fertilization,  as  follows : 

Syromastes Egg  n  +  spermatozoon  n  =  22(6"  or  9) 

Pyrrochoris  ...Egg  12  +  spermatozoon  12  =  24(d1or  9) 

whereas  in  Anasa  or  Protenor  the  relations  are: 

Anasa Egg  u  +  spermatozoon  10  =  21  (c?) 

Egg  ii  +  spermatozoon  n  =  22 (  9) 
Protenor Egg    7  +  spermatozoon    6  =  13  (c?) 

Egg    7  +  spermatozoon    7  =  14(9) 

In  the  hope  of  clearing  up  this  perplexing  contradiction  I 
endeavored  to  procure  material  for  a  reexamination  of  the  two 
forms  in  question,  and  through  the  great  kindness  of  Professor 
Boveri,  to  whom  my  best  thanks  are  due,  was  fortunate  enough  to 
obtain  an  abundant  supply  of  both,  though  unluckily  it  includes 
no  female  material.4  As  far  as  the  relations  can  be  worked  out  on 
the  male  alone  they  give,  I  believe,  the  solution  of  the  puzzle  and 
bring  the  two  species  in  question  into  line  with  the  general  princi- 
ple that  has  been  established  for  other  forms.  This  is  evidently 
true  of  Pyrrochoris.  i  Syromastes,  however,  constitutes  a  new 

3  At  first  thought  this  seems  to  be  in  harmony  with  the  remarkable  discovery  of  Meves  ('03,  '07)  that 
in  the  male  honey  bee  actual  polar  bodies  are  formed  which  produce  abortive  spermatids.    Butobviously 
the  two  cases  are  not  parallel,  for  in  the  bee  the  fertilized  eggs  produce  only  females;  and  this  finds  a 
natural  explanation,  in  accordance  with  the  general  conclusions  of  McClung,  Stevens  and  myself,  in  the 
assumption  that  it  is  the  male  producing  class  that  degenerate  as  polar  bodies. 

4  The  material,  fixed  in  Flemming's  fluid  and  in  Bouin's  picro-acetic-formol  mixture,  is  of  excellent 
quality  and  gave  preparations  of  perfect  clearness.     The  Flemming  material  is  on  the  whole  the  best. 
For  single  stains  Zwaardemaker's  safranin  and  iron  haematoxylin  were  employed  (the  latter   especially 
for  photographs).     Various  double  stains  were  also  used.     One  of  the  best,  which  I  can  strongly  recom- 
mend to  other  workers  in  this  field,  is  the  combination  of  safranin  and  lichtgriin,  which  gives  prepara- 
tions of  admirable  clearness  and  is  also  easy  to  use  and  certain  in  its  results. 


72  Edmund  B.    Wilson 

type  that  is  not  yet  known  to  be  exactly  paralleled  in  other 
forms;  though,  as  will  appear,  the  genus  Galgulus  presents  a 
somewhat  analogous  case.  It  does  not  seem  to  have  occurred  to 
Dr.  Gross  (as  it  did  not  to  me  until  I  had  carefully  studied  both 
forms)  that  Syromastes  and  Pyrrochoris  might  be  of  different 
type,  but  such  is  evidently  the  case.  I  shall  endeavor  to  show  that 
Pyrrochoris  is  of  quite  orthodox  type,  having  an  odd  somatic 
number  in  the  male  and  a  typical  unpaired  idiochromosome. 
Since  I  am  compelled  to  differ  with  Dr.  Gross  in  regard  to  this 
species,  I  am  glad  to  admit  that  the  doubts  I  formerly  expressed 
as  to  his  account  of  the  spermatogonial  number  in  Syromastes, 
were  unfounded.  In  regard  to  the  female  number,  on  the  other 
hand,  I  believe  he  was  misled  by  a  wrong  theoretic  expectation 
(as  he  evidently  was  in  case  of  the  male  Pyrrochoris) ,  though  it 
is  possible  that  his  determination  of  the  apparent  number  was 
also  correct,  as  indicated  beyond. 

SYROMASTES    MARGINATUS    L. 

Gross's  account  of  this  form  was  as  follows:  The  somatic 
groups  in  both  sexes  are  stated  to  show  twenty-two  chromosomes. 
The  "  accessory"  chromosome  arises  by  the  synapsis  of  two 
spermatogonial  chromosomes,  and  is  therefore  a  bivalent.  It 
divides  equationally  in  the  first  spermatocyte  division  but  fails  to 
divide  in  the  second,  passing  bodily  to  one  pole  in  advance  of  the 
other  chromosomes  without  even  entering  the  equatorial  plate. 
All  of  the  spermatid-nuclei  thus  receive  ten  chromosomes  and 
half  of  them  in  addition  the  "accessory."  These  are  the  essen- 
tial conclusions;  but  they  are  complicated  by  the  following  singular 
view  of  the  relations  between  the  "accessory"  and  the  micro- 
chromosomes  or  "m-chromosomes."  The  chromosome  nucleolus 
of  the  growth  period  is  supposed  not  to  give  rise  (as  it  does  in 
Pyrrochoris  and  other  forms)  to  the  heterotropic  or  "accessory" 
chromosome  of  the  spermatocyte  divisions,  but  to  the  m-chromo- 
some  bivalent — the  same  view  as  the  earlier  one  of  Paulmier 
('99)  which  has  since  been  shown  to  be  erroneous  (Wilson  '05 c). 
But,  on  the  other  hand  it  is  believed  to  arise,  not  from  the 


Studies  on  Chromosomes  73 

ra-chromosomes  of  the  spermatogonia,  but  from  two  larger  chro- 
mosomes, while  the  spermatogonial  ra-chromosomes  are  supposed 
to  be  converted  into  the  "accessory"  (!).  I  will  not  enter  upon  the 
very  ingenious,  if  somewhat  fantastic,  conclusions  that  are  based 
on  these  results,  for,  as  I  shall  attempt  to  show,  the  results  them- 
selves cannot  be  sustained  in  some  important  particulars.  But 
apart  from  this  I  am  glad  to  be  able  to  give  the  most  positive  con- 
firmation of  Gross's  interesting  discovery  in  regard  to  the  numer- 
ical relations  in  the  male.  Syromastes  is  indeed  a  case  in  which 
the  spermatogonial  number  is  an  even  one  (twenty-two),  while  there  is 
a  heterotropic  chromosome  in  the  second  division.  Half  the  sperma- 
tozoa seem  to  receive  ten  chromosomes  and  half  eleven,  as  in  so 
many  other  species  of  Coreidae.  But  as  Gross  also  correctly  de- 
scribed, the  heterotropic  chromosome  is  here  a  bivalent  which 
represents  two  chromosomes  united  together.  The  true  numbers 
characteristic  of  the  two  classes  of  spermatozoa  are  therefore 
ten  and  twelve,  respectively.  For  the  sake  of  clearness  I  will  here 
point  out  that  this  becomes  at  once  intelligible  under  the  assump- 
tion that  the  female  number  is  not  twenty-two,  as  Gross  believed, 
but  twenty-four;  and  such  I  believe  will  be  found  to  be  the  fact. 
That  Gross  was  mistaken — doubtless  misled  by  the  earlier 
conclusion  of  Paulmier  ('99),  in  which  he  was  at  first  followed  by 
Montgomery  ('01) — in  supposing  that  the  chromosome  nucleolus 
of  the  growth  period  divides  to  form  the  m-chromosomes,  is  I 
think  thoroughly  demonstrated  by  my  preparations.  In  the  case 
of  Anasa  and  Alydus  I  showed  ('o5c)  that  the  m-chromosomes  are 
not  formed  in  the  way  Paulmier  believed,  but  arise  from  two  small 
separate  rod-like  chromosomes  that  are  in  a  diffused  condition 
during  the  growth  period  and  only  condense  to  form  compact 
bodies  at  the  same  time  that  the  condensation  of  the  larger  chro- 
mosomes takes  place.  I  have  since  found  this  to  be  true  of  many 
other  species.  It  is  confirmed  in  the  case  of  Anasa  by  the  smear 
preparations  of  Foot  and  Strobell  ('07),  and  I  have  also  since  fully 
established  the  same  conclusion  by  this  method,  by  means  of 
which  every  chromosome  in  the  nucleus  may  be  demonstrated.5 

5  This  is  opposed  to  the  conclusion  of  Montgomery  ('06). 


74  Edmund  B.    Wilson 

Although  I  have  no  smear  preparations  of  Syromastes  it  is  perfectly 
clear  from  the  sections  that  the  facts  are  the  same  here  as  in  Anasa 
Alydus,  and  other  forms.  In  the  early  prophases  of  the  first  divi- 
sion (at  a  period  corresponding  to  Gross's  Figs.  31  to  37)  when  the 
plasmosome  has  disappeared  or  is  greatly  reduced  in  size,  the 
nuclei  contain  both  the  chromosome-nucleolus  and  the  m-chro- 
mosomes.  This  is  shown  in  great  numbers  of  cells  with  unmis- 
takable clearness  and  after  various  methods  of  staining,  particu- 
larly after  safranin  alone  or  combined  with  lichtgriin.  In  the 
early  part  of  this  period  the  chromosome  nucleolus  is  at  once 
recognizable  by  its  intense  color  and  sharp  contour  and  is  not 
for  a  moment  to  be  confused  with  a  plasmosome.  The  ordinary 
bivalents  are  still  in  the  form  of  ragged  pale  bodies,  having  the 
form  of  longitudinally  split  rods  or  double  crosses.  The  m-chro- 
mosomes  have  the  same  texture  and  staining  reaction,  but  are 
much  smaller  and  never  show  the  cross  form.  While  it  is  diffi- 
cult to  show  the  facts  to  demonstration  in  photographs  of  sections 
they  may  be  fairly  well  seen  in  the  following.  Photo  18  shows  the 
chromosome  nucleolus  (not  quite  in  focus,)  one  of  the  large  biva- 
lents (two  others  barely  appear)  and  both  m-chromosomes. 
Photo  19  is  a  similar  view  (the  m-chromosomes  more  condensed), 
while  Photo  20  shows  the  m-chromosomes  and  three  of  the  ordi- 
nary bivalents.  The  succeeding  changes  must  be  rapidly  passed 
through,  since  the  successive  steps  are  often  seen  in  the  same  cyst, 
passing  from  one  side  to  the  other.  In  these  stages  the  large 
bivalents  rapidly  condense  and  regain  their  staining  capacity, 
finally  assuming  a  bipartite  or  quadripartite  form.  The  m-chro- 
mosomes undergo  a  similar  condensation,  being  finally  reduced  to 
ovoidal  or  spheroidal  bodies.  The  chromosome  nucleolus,  on 
the  other  hand,  becomes  somewhat  looser  in  texture  and  assumes 
an  asymmetrical  quadripartite  shape,  in  which  form  it  enters  the 
equatorial  plate  to  form  the  eccentric  "accessory"  chromosome. 
The  period  at  which  the  m-chromosomes  condense  varies  consider- 
ably, and  the  same  is  true  of  their  relative  position;  sometimes  they 
are  in  contact,  sometimes  more  or  less  widely  separated,  even  lying 
on  opposite  sides  of  the  nucleus.  Photo  21  shows  two  nuclei, 
one  above  the  other,  in  each  of  which  appear  both  m-chromosomes, 


Studies  on  Chromosomes  75 

the  chromosome  nucleolus  and  a  number  of  the  other  bivalents. 
Photo  22  shows  the  same  condition.  Photo  23,  from  the  same  cyst, 
is  slightly  later,  showing  the  two  spheroidal  ra-chromosomes  wide 
apart,  the  chromosome  nucleolus,  and  several  of  the  other  chro- 
mosomes. (The  chromosomes  nucleolus,  perfectly  recognizable 
in  the  preparation,  is  in  the  photograph  hardly  distinguishable 
from  the  other  bivalents  seen  endwise.)  Up  to  this  point,  which 
shortly  precedes  the  dissolution  of  the  nuclear  membrane,  the 
chromosome  nucleolus  is  still  immediately  recognizable  by  its 
deeper  color  (especially  after  safranin).  There  follows  a  brief 
period  in  which  this  distinction  disappears,  but  the  chromosome 
nucleolus  is  still  recognizable  by  its  asymmetrical  form.  That  it 
gives  rise  to  the  eccentric  "accessory"  is,  I  think,  beyond  doubt. 
The  evidence  is  demonstrative  that  it  does  not  divide  to  form  the 
m-chromosomes,  and  that  the  latter  arise  from  separate  rods  as 
described.  Gross  appears  to  have  seen  these  rods  at  a  much 
earlier  period  (cf.  his  Fig.  10)  and  correctly  identifies  them  with 
the  spermatogonial  m-chromosomes;  but  he  believed  them  to  give 
rise  to  the  "accessory." 

The  relation  of  the  chromosome  nucleolus  to  the  spermato- 
gonial chromosomes  cannot  be  determined  in  Syromastes  with  the 
same  degree  of  certainty  as  in  Pyrrochoris  (as  described  beyond) , 
but  the  size  relations  leave  hardly  a  doubt  that  Gross  was  right 
in  asserting  its  origin  from  two  of  the  larger  of  these  chromosomes. 
The  study  of  these  relations  is  of  importance  because  I  believe 
they  justify  the  conclusion  that  the  chromosome  nucleolus,  and 
hence  the  "accessory,"  is  nothing  other  than  a  pair  of  slightly 
unequal  idiochromosomes,  which  can  readily  be  recognized  in  the 
spermatogonial  groups. 

Study  of  the  spermatogonial  groups  in  detail  shows  that  twenty 
of  the  chromosomes  may  be  equally  paired,  while  the  remaining 
two  are  slightly  but  distinctly  unequal  in  size.  These  can  always 
be  recognized  as  the  smallest  of  the  chromosomes  except  the 
m-chromosome .  Photos  I  and  2  show  two  groups  in  which  this 
clearly  appears.  These  photographs  are  reproduced  in  Text  Figs. 
i<3,  ib,  with  two  others,  c  and  d,  the  chromosomes  in  question 
being  designated  as  I  and  /'. 


76 


Edmund  B.    Wilson 


It  is  evidently  this  pair  that  give  rise  to  the  bivalent  "accessory" 
(eccentric)  chromosome  of  the  first  division  and  hence  to  the 
chromosome  nucleolus  of  the  growth  period.  Gross  correctly 
describes  this  bivalent  as  a  quadripartite  body  or  tetrad,  but 
overlooked  the  fact  that  it  is  composed  of  two  slightly  unequal 
halves,  and  these  correspond  in  relative  size  to  the  unequal  pair 
in  the  spermatogonia.  This  appears  unmistakably  in  a  great 
number  of  polar  views  of  the  first  division  metaphase  (though  it 
is  not  always  apparent)  and  is  clearly  shown  in  Photos  3,  4  and 
<J.  It  is  evident  that  the  bivalent  is  so  placed  in  the  equatorial 


a 


I 


FIG.  I.  Four  spermatogonial  chromosome-groups  of  Syromastes  marginatus;  a  and  b  are  reproduc- 
tions of  Photos  i  and  2.* 

*  The  drawings  are  not  made  from  the  microscope  with  the  camera  lucida  but  directly  upon  enlarged 
photographs  of  the  objects.  Since  I  believe  this  method  to  be  superior  in  accuracy  for  the  representa- 
tion of  such  small  objects  I  will  briefly  describe  it  in  the  hope  that  others  may  find  it  useful.  The 
original  negatives  are  taken  directly  from  the  sections  at  an  enlargement  of  1500  diameters  (2  mm.  oil 
immersion,  compensation  ocular  6).  From  these  negatives  enlarged  bromide  prints  are  made  (with  a 
photographic  camera)  three  times  the  size  of  the  original  negatives  (i.  e.,  4500  diameters)  upon  double 
weight  paper,  which  gives  a  good  surf  ace  for  pen  drawings.  The  drawing  is  then  made  directly  on  the 
print  with  waterproof  ink,  and  when  thoroughly  dry  the  remains  of  the  photograph  are  bleached  out  in 
a  mixture  of  sodium  hyposulphite  and  potassium  ferricyanide.  The  enlarged  prints  of  course  show  the 
chromosomes  with  more  or  less  blurred  outlines  (though  they  are  clearer  than  might  be  supposed); 
but  by  working  with  an  ordinary  print  and  the  object  before  one  for  comparison  the  drawings  may 
nevertheless  be  made  with  great  accuracy.  They  may  be  tested  and  if  necessary  corrected,  by  the  use 
of  a  reducing  glass. 


Studies  on  Chromosomes  77 

plate  as  to  undergo  an  equation  division,  like  the  idiochromosomes 
of  other  Hemiptera  heteroptera.  In  uniting  to  form  a  bivalent 
before  the  first  division  these  chromosomes  differ  from  those  of 
most  other  Hemiptera,  but  in  all  other  respects  up  to  the  end  of 
the  first  division  they  correspond  exactly  with  them.  But  even 
this  difference  is  bridged  by  a  condition  occasionally  seen  in  other 
forms,  for  instance  in  Lygaeus  and  Metapodius.6  In  the  last 
named  form  the  typical  and  usual  condition  is  that  the  idiochro- 
mosomes are  in  the  first  division  quite  separate,  lying  eccentrically 
outside  the  principal  ring  of  chromosomes  like  the  unpaired  idio- 
chromosomes of  other  coreids  (Photos  6  and  7),  and  in  this  posi- 
tion they  separately  divide.  Exceptionally,  however,  they  lie  in 
close  contact  (Photos  8  and  9),  forming  an  asymmetrical  bivalent 
precisely  like  that  of  Syromastes.  In  both  cases  this  bivalent 
divides  equationally,  giving  two  asymmetrical  daughter-dyads, 

thus  g. 

The  exactness  of  the  correspondence  up  to  this  point  seems  to 
leave  no  doubt  of  the  homology  of  this  pair  of  chromosomes  in  the 
two  forms.  In  the  second  division,  however,  the  two  species  show 
a  remarkable  contrast.  In  Metapodius,  as  in  Lygaeus  or  Euschis- 
tus,  the  two  idiochromosomes  are  always  united  to  form  an  unsym- 
metrical  bivalent  which  enters  the  equatorial  plate  and  is  separated 
into  its  two  components,  half  the  spermatids  receiving  the  large 
one  and  half  the  small.  In  Syromastes,  on  the  other  hand,  the 
idiochromosomes  remain  united  and  do  not  enter  the  equatorial 
plate  at  all,  but  pass  directly  to  one  pole  where  they  are  included 
in  the  daughter-nucleus,  as  Gross  has  described  (Photos  II  to 
17).  Owing  to  this  behavior  of  the  idiochromosome  bivalent, 
polar  views  of  the  second  division  always  show  but  ten  chromo- 
somes instead  of  eleven  (Photo  10).  In  this  case  therefore  half 
the  spermatid  nuclei  receive  two  more  chromosomes  than  the 
others,  the  two  classes  having  respectively  ten  and  twelve  chromo- 
somes. As  the  idiochromosome  bivalent  passes  to  the  pole  its 
two  components  are  usually  closely  united,  and  often  cannot  be 

6  The  latter  remarkable  genus,  which  presents  the  phenomenon  of  the  "supernumerary  chromosomes" 
(Wilson  'oyc),  will  form  the  subject  of  a  forthcoming  fifth  "Study." 


78  Edmund  B,    Wilson 

distinguished;  but  in  some  cases  they  may  still  be  seen,  as  in  Photo 
12.  As  the  nuclear  vacuole  forms  the  ordinary  chromosomes 
rapidly  diminish  in  staining  capacity,  while  the  idiochromosome 
bivalent  retains  its  compact  form  and  dark  color,  like  a  nucleolus, 
and  thus  comes  conspicuously  into  view,  particularly  after  safranin. 
Its  double  nature  is  at  this  time  often  more  clearly  apparent  than 
in  the  preceding  stages.  It  disappears  from  view  some  time  after 
the  reconstruction,  at  a  much  earlier  period  than  in  Pyrrochoris. 

Only  exceptionally  in  my  preparations  do  the  chromosomes  of 
the  second  division  show  a  quadripartite  form  as  Gross  figures 
them.  Their  usual  form  is  dumb-bell  shaped  or  dyad-like; 
though  as  the  two  halves  separate  they  are  often  connected  by 
double  fibers,  as  is  the  case  with  many  other  species  of  Hemiptera. 


PYRROCHORIS  APTERUS  L. 


As  already  stated,  Pyrrochoris  is  of  different  type  from  Syro- 
mastes  and  agrees  precisely  with  other  forms  having  an  unpaired 
idiochromosome,  such  as  Anasa  or  Protenor.  Aside  from  the 
interest  that  this  species  possess  as  the  one  in  which  Henking  first 
discovered  the  idiochromosome,  it  is  in  other  respects  a  peculiarly 
interesting  form  for  the  study  of  the  general  spermatogenesis, 
particularly  in  respect  to  the  presynaptic  and  synaptic  periods. 
I  shall  here,  however,  confine  myself  mainly  to  the  numerical 
relations  and  the  history  of  the  idiochromosome.  Henking  him- 
self somewhat  doubtfully  concluded  that  the  spermatogonial 
number  was  twenty-four:  "Ich  habe  in  drei  Fallen  die  Zahl  24 
erhalten,  in  einem  Falle  die  Zahl  23.  Da  die  Bilder  iiberall  die 
gleichen  sind,  so  habe  ich  das  Zahlgeschaft  nicht  an  einer  grosseren 
Zahl  vorgenommen  und  glaube  die  theoretisch  zu  erwartende  Zahl 
24  als  das  Normal  ansehen  diirfen"  (op.  cit.,  p.  688,  italics  mine). 
It  is  clear  enough  from  this  that  Henking,  too,  was  misled  by  a 
false  theoretic  expectation;  and  a  study  of  his  figures  (op.  cit., 
Figs.  6,  a,  b.  c,  7)  will  show  that  they  are  very  far  from  decisive. 
In  the  case  of  the  female,  Henking  speaks  much  more  positively 
('92)  and  there  is  hardly  a  doubt  that  his  count  of  twenty-four 
chromosomes  was  correct,  since  he  found  this  number  "unverkenn- 


Studies  on  Chromosomes  79 

bar"  in  the  dividing  oogonia,  and  in  the  connective  tissue  cells  of 
the  ovary,  and  also  figured  (Fig.  39)  a  double  group  (exactly  such 
as  I  described  in  Anasa,  Wilson  '06,  Fig.  2,£),  showing  forty-eight 
chromosomes. 

Gross  accepts  Henking's  account  without  question,  treating  the 
numerical  relations  in  rather  summary  fashion  as  follows:  "Die 
Aequatorialplatte  der  sich  teilenden  Spermatogonien  enthalt  24 
Chromosomen.  Dieselbe  Zahl  hat  Henking  ausser  in  den  Sperma- 
togonien auch  in  den  Oogonien  gefunden.  Ebenso  konnte  ich  in 
den  Follikelzellen  der  Eirohren,  also  in  somatischen  Zellen,  kon- 
statieren.  24  ist  also  die  Normalzahl  der  Species"  ('07,  p.  277). 
In  support  of  this  are  given  two  polar  views  of  spermatogonial 
metaphases  (the  female  groups  are  not  figured)  each  showing  eight 
small  and  sixteen  large  chromosomes  (Figs.  9  and  10).  His  ac- 
count continues  as  follows :  The  idiochromosome  appears  aready  in 
the  syanaptic  period  (synizesis)  as  a  double  nucleolus-like  body, 
assumed  to  be  a  bivalent  body  that  arises  by  the  synapsis  of  two 
of  the  spermatogonial  chromosomes,  though  none  of  the  earlier 
stages  were  followed  out.  At  a  later  period  it  appears  as  a  single 
spheroidal  body  owing  to  the  close  apposition  of  its  two  halves. 
This  chromosome  divides  in  the  first  spermatocyte  division,  but 
in  the  second  lags  behind  the  others  and  passes  undivided  to  one 
pole,  as  Henking  described.  All  of  the  spermatid-nuclei  thus 
receive  eleven  chromosomes,  while  half  of  them  receive  in  addi- 
tion the  idiochromosome.  Since  both  sexes  were  supposed  to  con- 
tain twenty-four  chromosomes,  Gross  drew  the  same  conclusion 
as  the  one  previously  reached  in  the  case  of  Syromastes,  namely, 
that  only  the  twelve-chromosome  spermatozoa  are  functional. 

In  regard  to  the  spermatocyte  divisions  my  own  results  are 
perfectly  in  accord  with  Henking's  and  Gross's.  As  to  the 
spermatogonial  number,  I  must  say  that  after  having  immediately 
confirmed  Gross's  account  of  Syromastes  (which  I  examined  first) 
I  was  fully  prepared  to  find  a  similar  relation  in  Pyrrochoris.  It 
was  therefore  with. astonishment  that  I  found  everywhere  twenty- 
three  instead  of  twenty-four  spermatogonial  chromosomes.  This 
number  appears  with  diagrammatic  clearness  in  a  great  number  of 
spermatogonia  from  different  individuals  (testes  from  35  different 


8o 


Edmund  B.    Wilson 


individuals  have  been  sectioned)  and  is  shown  both  in  camera 
drawings  and  in  photographs.  Eight  of  the  latter  are  shown 
(Photos  24  to  31),  and  these  same  groups  are  also  represented  in 
the  drawings,  Text  Figs.  2,  a,  b,  c,  d,  e,  j,  k,  /,  together  with  four 
others  (/,  g,  h,  z),  also  from  photographs.  Inspection  of  these 
photographs  and  drawings  will  show  that  the  unpaired  idiochro- 
mosome  is  at  once  recognizable  by  its  large  size,  which  renders 


FIG.  i.  Spermatogonial  groups  of  Pyrrochoris  apterus  (drawn  on  photographic  enlargements,  as 
explained  under  Fig.  i);  a,  b,  c,  d,  e,  j,  k  and  /  are  reproductions  of  Photos  24,  25,  26,  27,  28,  29,  30 
and  31,  respectively. 

it  almost  as  conspicuous  as  in  Protenor  (heretofore  described  by 
Montgomery  and  myself).  I  find  the  size  relations  not  quite  the 
same  as  Gross  describes  them.  There  are,  as  he  states,  eight 
chromosomes  that  are  considerably  smaller  than  the  others;  but 
two  of  the  others  are  but  slightly  larger.  The  remaining  twelve 
paired  chromosomes  are  much  larger,  though  the  contrast  is 
i  n  my  material  not  so  great  as  Gross  figures  it.  The  idiochromo- 


Studies  on  Chromosomes  8 1 

some  is  nearly  twice  as  large  as  any  of  the  others,  and  is  obviously 
unpaired.  I  have  examined  a  large  number  of  spermatogonial 
groups  with  great  care  with  a  view  to  the  possibility  that  this  chro- 
mosome might  in  reality  be  double,  but  am  thoroughly  convinced 
that  such  is  not  the  case.  This  is  unmistakably  evident  when  this 
chromosome  has  the  form  of  a  straight  or  only  slightly  curved  rod 
(Photos  24  to  28,  Text  Fig.  2,  a  to  /),  and  these  constitute  the 
great  majority  of  observed  cases.  I  have,  however,  found  a  few 
cases  where  it  has  a  very  marked  sigmoid  curvature;  two  or  three 
of  these  give  at  first  sight  the  appearance  of  two  chromosomes  in 
contact  (Photos  29,  30,  31 ;  Text  Fig.  2,  /,  k,  /),  Even  here  close 
study  shows  that  it  is  a  single  body;  but  such  forms  might  readily 
mislead  an  observer  having  a  preconceived  idea  of  the  number  to 
be  expected. 

That  this  is  a  single  chromosome  that  is  identical  with  the  idio- 
chromosome  of  the  growth  period  and  the  maturation  divisions 
is  placed  beyond  doubt  by  a  study  of  the  presynaptic  stages, 
which  were  not  examined  by  either  Henking  or  Gross.  This 
period  is  of  such  interest  in  Pyrrochoris  as  to  merit  a  special  study. 
With  only  a  single  exception  I  know  of  no  other  form  in  which  the 
history  of  the  idiochromosome  and  the  succession  of  the  stages  can 
be  so  completely  and  readily  followed  at  this  time.  Throughout 
this  whole  period,  beginning  with  the  telophases  of  the  last  sperma- 
togonial division,  the  idiochromosome  can  be  traced  step  by  step 
as  a  single  body,  and  it  is  evidently  identical  with  the  large  un- 
paired spermatogonial  chromosome. 

In  the  stages  that  immediately  follow  the  last  spermatogonial 
telophase  (Photos  32  and  33)  the  chromosomes  still  retain  their 
boundaries,  though  they  show  a  looser  texture,  vaguer  outlines 
and  diminished  staining  capacity  (by  which  characters  the  post- 
phases  are  readily  distinguishable  from  the  prophases).  The  large 
chromosome  (idiochromosome)  is  clearly  distinguishable  at  this 
time,  both  by  its  size  and  by  its  deeper  color.  In  the  stages  that 
immediately  follow  a  remarkable  contrast  appears  between  this 
chromosome  and  the  others.  The  latter  rapidly  lose  their  visible 
boundaries  and  their  staining  capacity,  breaking  up  into  a  fine  net- 
like  structure  in  which  traces  of  a  spireme-like  arrangement  may 


82  Edmund  B.    Wilson 

sometimes  be  seen.     The  idiochromosome,  on  the  other  hand, 
retains  its  identity  and  deep  color  and  now  appears  as  a  conspicu- 
ous elongated  body  ("caterpillar  stage").      Though  its  outlines 
are    still    somewhat   ragged    and  its  color  less  intense    than   in 
the   succeeding  stages,  it  already  appears  in   sharp   contrast   to 
the  pale  reticulum  (Photos  34  and   35).      It  sometimes  extends 
straight  across  the  whole  diameter  of  the  nucleus;  but  beside  such 
forms,  in  the  same  cysts,  are  often  curved  and  shorter   forms. 
At  this  time  it  is  usually  surrounded  by  a  distinct  clear  space  or 
vacuole,  as  I  hope  the  photographs  may  show;  and  there  are  also 
in  the  nucleus  from  one  to  three  much  smaller  nucleolus-like  bodies 
which  (on  account  of  the  staining  reactions)  I  believe  to  be  plas- 
mosomes,  but  these  soon  disappear.     Splendid  pictures  of  these 
and  the  following    stages    are  given  by    the    safranin-lichtgriin 
combination,  which  shows  the   idiochromosome   at  every  stage 
bright  red,  while  in  properly  differentiated  preparations  the  reticu- 
lum   is   pure  green.7     The  idiochromosome  now  takes  up  a  pe- 
ripheral position  and  the  clear  space  surrounding  it  disappears. 
It  acquires  a  more  definite  contour,  stains  still  more  intensely, 
and  rapidly  shortens  until  it  is  converted  into  a  condensed  ovoidal 
or  spheroidal  chromosome  nucleolus  that  may  be  traced  without 
a  break  through  every  stage  up  to  the  prophases  of  the  first  sperma- 
tocyte  division.     As  it  shortens  it  may  undergo  a  variety  of  form 
changes.     In  what  I  regard  as  the  typical  process  it  shows  no 
indication  of  duality  at  any  period  up  to  the  full  contraction  phase 
(synizesis)   being  progressively  reduced  to  a  short  rod  and  finally 
to  an  ovoidal  or  spheroidal  body  (Photos  36  to  42).     In  the  mean- 
time  the   nuclear  reticulum   contracts   more   and   more,   usually 
towards  one  side  of  the  nucleus,  becomes  coarser  in  texture,  and 
increases  in  staining  capacity,  until  at  the  climax  of  the  process 

7  The  effect  of  this  stain  depends  in  some  measure,  of  course,  on  the  relative  degree  of  extraction  of 
the  two  dyes.  My  method  is  to  stain  in  safranin  for  two  to  four  hours  and  then  to  place  the  slide  at 
once  in  strong  alcoholic  solution  of  lichtgriin  for  ten  to  twenty  seconds.  This  is  at  once  followed  by 
rapid  washing  in  95  per  cent  and  absolute  alcohols.  The  alcohol  is  then  replaced  by  clove  oil  and  the 
latter  by  xylol.  In  all  cases  the  chromosomes  of  dividing  cells  and  the  chromosome  nucleolus  of  all 
stages  appear  brilliant  red,  the  achromatic  fibers  and  general  cytoplasm  pure  green.  The  relative 
intensity  of  red  and  green  depend  on  the  length  of  immersion  in  the  green  solution.  The  description 
here  given  applies  to  sections  rather  strongly  stained  in  the  green. 


Studies  on  Chromosomes  83 

it  forms  a  close  knot,  or  rounded  mass,  staining  almost  black  in 
haematoxylin,  at  one  side  of  which  is  the  idiochromosome  (now  a 
condensed  chromosome  nucleolus).  These  structures  lie  in  a 
large  clear  nuclear  vacuole,  as  shown  in  Photos  39  to  42.  The 
stage  thus  attained  is  the  characteristic  contraction  phase  or 
synizesis,  which  in  this  species  is  extremely  marked.8 

In  the  safranin-lichtgriin  preparation  at  this  period  the  chro- 
mosome nucleolus  is,  as  always,  intensely  red.  The  synaptic 
knot  varies  with  the  relative  intensities  of  the  red  and  green,  being 
in  some  preparations  distinctly  red,  in  others  pure  green,  in  still 
others  of  mixed  appearance.  In  the  succeeding  stage  the  chromatin 
emerges  from  the  synaptic  knot  in  the  form  of  separate  spireme 
threads  which  lose  their  staining  capacity  for  haematoxylin  and 
in  the  double  stain  are  again  pure  green  (Photos  43  and  44). 
In  the  middle  and  late  growth  period  they  are  still  more  or  less 
green  but  contain  red  granules.  In  the  prophases  of  the  first  divi- 
sion they  at  last  lose  their  affinity  for  the  green  and  finally  appear 
pure  red;  but  this  does  not  occur  until  just  before  the  dissolution 
of  the  nuclear  membrane.  Since  the  idiochromosome  always 
retains  its  intense  red  color  it  may  thus  be  followed  from  stage  to 
stage  with  great  certainty. 

The  study  of  the  whole  cycle  of  changes  from  the  last  sper- 
matogonial  division  onward  gives  certain  very  definite  results  in 
regard  to  synapsis  in  general,  and  especially  in  regard  to  the  idio- 

8  Many  recent  writers  have  expressed  the  opinion  that  the  synizesis  stage  is  an  artifact  produced  as 
a  shrinkage  product,  though  Miss  Sargant  ('96)  stated  very  explicitly  that  she  had  seen  it  in  the  living 
cells,  and  this  has  recently  been  confirmed  by  Overton  ('05).  I  can  fully  substantiate  this  in  the  case 
of  Anasa  tristis.  The  perfectly  fresh  testis,  gently  teased  apart  in  a  Ringer's  fluid  in  which  the  sperma- 
tozoa continue  their  active  movements,  very  clearly  shows  nearly  all  the  features  of  the  spermatogenesis, 
including  the  number,  shape  and  size  relations  of  the  chromosomes,  their  characteristic  grouping  and 
behavior  in  the  spermatocyte  divisions,  the  double  rods,  crosses  and  other  prophase  figures,  the  spindle 
fibers  and  asters,  and  even,  I  believe,  the  centrosomes.  In  this  fresh  material  the  synizesis  stage 
appears  in  essentially  the  same  form  as  in  the  sections,  the  nuclear  knot  lying  in  a  large  clear  vacuole. 
These  nuclei  only  appear  in  the  same  region  of  the  testis  as  in  sections,  and  they  show  a  conspicuous 
contrast  to  those  of  earlier  and  later  stages  that  lie  near  by  them.  In  the  post  synaptic  stages  the  chromo- 
somes, in  the  form  of  spireme  threads  can  be  seen  again  spreading  through  the  nuclear  cavity.  These 
observations  leave  no  doubt  in  my  mind  that  the  synizesis  is  a  normal  phase  of  the  spermatogenesis  in 
these  animals,  though  it  is  not  improbable  that  the  contraction  may  be  somewhat  exaggerated  by  the 
reagents.  It  is  evident,  however,  from  such  studies  as  those  of  the  Schreiners  ('06)  and  others  that 
the  synizesis  does  not  occur  in  some  forms. 


84  Edmund  B.    Wilson 

chromosome.  Concerning  the  first  point  I  will  here  only  indicate 
one  principal  conclusion.  It  is  quite  clear  that  in  Pyrrochoris 
(and  I  think  the  same  holds  true  in  other  Hemiptera)  synapsis, 
or  the  conjugation  of  chromosomes  two  by  two,  does  not  occur  in 
the  closing  anaphases  of  the  last  spermatogonial  division  as  was 
described  by  Montgomery  (*oo)  in  Peripatus  and  Euschistus 
("Pentatoma"),  by  Sutton  ('02)  in  Brachystola,  by  Stevens  ('03) 
in  Sagitta,  and  by  Dublin  ('05)  in  Pedicellina.  Although  the 
number  of  chromosomes  in  the  postphases  immediately  following 
this  division  (Photos  32  and  33)  cannot  be  exactly  made  out,  it 
is  perfectly  evident  that  it  is  not  the  reduced  number  but  approxi- 
mates to  the  somatic  number  (twenty-three).  The  chromosomes, 
therefore,  have  not  paired  two  by  two  in  the  spermatogonial  ana- 
phases.  It  is  equally  certain  that  this  stage  does  not  pass  directly 
into  the  synizesis  but  is  separated  from  it  by  a  long  "  resting  period" 
(Photos  34  to  38) — as  is  demonstated  by  the  topographical 
relations  as  well  as  by  the  progressive  stages  of  the  idiochromo- 
some — in  which  the  ordinary  chromosomes  lose  their  sharp 
boundaries  and  their  affinity  for  nuclear  stains.  In  this  respect 
Pyrrochoris  shows  a  close  similarity  to  Tomopteris,  as  described 
by  the  Schreiners  ('06),  whose  original  preparations,  by  the 
kindness  of  Dr.  Schreiner,  I  have  had  opportunity  to  examine. 
This  comparison  has  convinced  me  that  synapsis  occurs  at  the 
same  period  in  both — whether  by  parasynapsis  (side  to  side  union) 
or  telosynapsis  (end  to  end  union9)  or  in  some  other  way  I  am 
not  prepared  to  say.  There  can  be  no  manner  of  doubt  that 
the  first  division  of  the  bivalents  is  a  transverse  one,  as  described 
by  Paulmier  and  Montgomery;  but  it  has  been  rendered  evident 
enough  by  recent  studies  on  reduction  that  this  in  itself  gives  no 
trustworthy  evidence  regarding  the  mode  of  synapsis.  The 
direct  investigation  of  the  process  in  the  Hemiptera  presents 
great  difficulties. 

The  foregoing  general  conclusion  regarding  the  time  of  synapsis 
is  of  importance  for  the  more  specific  one  in  regard  to  the  idio- 
chromosome.  During  the  entire  earlier  presynaptic  period  the 

8  I  have  for  some  years  made  use  of  these  terms  in  my  lectures  on  cytology. 


Studies  on  Chromosomes  85 

elongated  idiochromosome  is  manifestly  a  single  body.  As  it 
shortens  and  condenses  to  form  the  chromosome  nucleolus,  it 
shows  a  considerable  variety  of  forms;  and  the  rate  of  condensation 
also  varies,  cells  that  are  already  enteringfthe  synizesis  stage  being 
sometimes  seen  in  which  the  idiochromosome  is  still  distinctly  a  rod 
(Photos  35  and  36).  In  most  cases  it  is  at  this  time  a  single 
ovoidal  or  spheroidal  body;  but  not  infrequently  it  appears  more  or 
less  distinctly  double  (Photos  37  to  39).  This  condition  is  however 
not  produced  by  a  previous  synapsis  of  two  chromosomes,  as  Gross 
believed,  but  arises,  I  think,  from  a  tendency  of  the  chromatin  to 
accumulate  towards  the  ends  of  the  rod;  and  when  this  is  very 
marked  it  may  assume  an  appearance  of  duality,  even  in  the  ear- 
lier stages  (Photo  37,  below),  though  this  is  relatively  rare.  In  the 
later  stages  a  double  appearance  is  not  infrequent,  dumb-bell 
forms  being  thus  produced,  which  sometimes  give  in  the  synizesis 
stage  apparently  double  bodies.  The  earlier  stages  conclusively 
show  that  this  is  a  secondary  appearance.  In  the  later  (postsyn- 
aptic)  stages  (Photos  34  and  35),  and  throughout  the  growth 
period,  it  always  appears  as  a  single  spheroidal  body.  In  view  of 
these  facts  I  think  the  conclusion  inevitable  that  the  chromosome 
nucleolus  is  a  univalent  chromosome  that  arises  by  the  condensa- 
tion of  the  unpaired  large  chromosome  of  the  spermatogonia. 

I  have  little  to  add  to  Henking's  and  Gross's  acounts  of  the 
maturation  divisions.  As  will  be  seen  from  Photos  45,  46,  50,  51, 
the  size  relations  are  correlated  with  those  of  the  spermatogonial 
chromosomes.  In  polar  views  of  the  first  division  appear  with 
great  constancy  four  smallest  chromosomes,  one  slightly  larger 
one,  and  seven  still  larger  ones,  or  twelve  in  all.  The  idiochromo- 
some is  one  of  the  largest,  but  cannot  be  distinguished  from  the 
others  (as  is  also  the  case  in  Protenor).  This  is  obviously  due  to 
the  fact  that  the  idiochromosome  is  still  a  univalent  or  single 
chromosome,  while  each  of  the  others  represents  two  of  the  sper- 
matogonial chromosomes  united.  Since  all  have  nearly  the,  same 
dumb-bell  shape  as  seen  in  side  view,  the  idiochromosome  appears 
from  the  pole  approximately  but  half  as  large,  relative  to  the  others, 
as  in  the  spermatogonia.  The  same  size-relations%  appear  in  the 
second  division,  but  all  the  chromosomes  are  much  smaller,  as 
the  photographs  clearly  show. 


86  Edmund  B.   Wilson 

I  have  not  succeeded  with  Pyrrochoris  (as  I  have  with  several 
other  genera)  in  obtaining  photographs  of  both  anaphase  daughter 
groups  showing  all  the  chromosomes;  but  it  is  perfectly  evident 
that  all  divide  equally  in  the  first  division,  and  all  but  the  idio- 
chromosome  in  the  second.  This  chromosome  lags  behind  the 
others  and  then  passes  undivided  to  one  pole  where  it  is  included 
in  the  daughter  nucleus  (Photos  47  to  49)  as  Henking  described  , 
This  pole  thus  receives  twelve  chromosomes,  the  other  but  eleven. 
As  in  a  number  of  other  species  the  idiochromosome  retains  its 
compact  form  and  deep-staining  capacity  long  after  the  reconstruc- 
tion of  the  nuclei  and  the  breaking  up  of  the  other  chromosomes. 
It  may  thus  be  distinguished  (especially  well  in  safranin  prepara- 
tions) up  to  a  rather  late  period  stage  of  the  spermatids,  even  after 
the  tails  have  grown  out.  It  finally  disappears  from  view,  and  the 
mature  spermatozoa  show  no  visible  indication  of  their  dimorphism. 

GENERAL 

If  my  conclusions  are  correct,  Pyrrochoris  agrees  exactly  with 
other  forms  in  which  an  unpaired  idiochromosome  is  present. 
Syromastes  however  presents  a  new  type  in  which  the  "accessory" 
chromosome  is  not  univalent  but  bivalent,  and  in  which  accord- 
ingly half  the  spermatozoa  receive  two  more  chromosomes  than 
the  other  half.  If  we  may  apply  the  same  rule  to  Syromastes  as 
that  which  holds  for  other  Hemiptera  we  may  expect  the  sperma- 
tozoa that  receive  the  "accessory"  to  be  female-producing,  the 
others  male-producing.  The  fertilization  formulas  for  the  two 
species  considered  in  this  paper  should  therefore  be  as  follows  : 


PYRROCHORIS 


Egg  12  +  spermatozoon  II  =  zyote  23  (c?) 
Egg  12  +  spermatozoon  12  =  zygote24(9) 


SYROMASTES 


Egg  12  (including  J  and  ;')*  +   spermatozoon      10  =  zygote  22  (including/  and  /')((?) 
Egg  12  (including  /and/)   +  spermatozoon      12    (including   /   and/)    =  zygote  24  (including 
/,  /,  »,  »,)(?) 

*  The  formation  of  a  reduced  female  group  of  this  composition  may  readily  be  explained  if  it  be  sup- 
posed that  in  syn  apsis  the  two  small  idiochromosomes  couple  with  each  other  to  form  the  bivalent 
/';',  the  two  large  ones  to  form  the  bivalent  II. 


Studies  on   Chromosomes  87 

The  correctness  of  my  deduction  may  readily  be  tested  by  a 
reexamination  of  the  female  groups.  Gross,  it  is  true,  states 
that  he  has  found  but  twenty-two  chromosomes  in  the  female 
(follicle  cells) ;  but  I  think  no  one  is  likely  to  consider  as  in  any 
way  conclusive  the  single  figure  that  he  gives  in  support  of  this 
(op.  cit.,  Fig.  in).  Not  less  than  five  of  the  twenty-two  chromo- 
somes figured  are  deeply  constricted;  and  any  one  of  these  might 
in  reality  be  two  chromosomes  in  contact.  I  hope  that  Dr. 
Gross  himself  may  be  willing  to  reexamine  this  point,  in  view  of 
the  possibility  here  suggested.  It  is  however  also  possible  that  the 
two  members  of  each  of  the  idiochromosome  pairs  in  the  female 
may  be  united  to  form  a  bivalent,  in  which  case  the  female  would 
apparently  show  but  twenty-two  chromosomes;  but  even  if  this 
be  so  the  two  members  must  separate  again  when  transferred  to 
the  male. 

In  regard  to  Pyrrochoris,  there  is  little  doubt  that  the  determina- 
tion of  the  female  number  by  Henking  and  Gross  as  twenty-four 
was  correct;  and  since  the  idiochromosome  in  the  male  is  the 
largest  of  the  chromosomes  we  may  expect  the  female  groups  to 
show  two  such  chromosomes.10 

I  should  state  the  expectation  less  confidently  in  the  case  of  Syro- 
mastes  if  it  stood  entirely  alone;  but  another  case  has  now  been 
made  known  in  which  the  male  and  female  groups  differ  by  more 
than  one  chromosome.  This  occurs  in  the  genus  Galgulus,  which 
has  been  worked  out  in  my  laboratory  by  Mr.  F.  Payne  (whose 
results  are  now  in  press)11  on  material  collected  by  myself.  The 
following  facts  are  very  clearly  shown  in  this  form.  The  sperma- 
togonial  number  is  thirty-five,  the  female  number  thirty-eight. 
In  the  second  division  five  of  the  chromosomes  are  always  asso- 

1U  Henking's  figures  ('92)  give  considerable  evidence  that  such  is  really  the  case.  His  Fig.  83  of 
the  first  polar  metaphase  shows  one  of  the  twelve  bivalents  fully  twice  the  size  of  the  others;  and  the 
same  is  true  of  Fig.  68,  which  shows  a  side  view  of  the  second  polar  spindle,  though  not  all  the  chromo- 
somes are  shown.  With  this  accords  his  Fig.  39  of  a  double  group  from  a  connective  tissue  cell  of 
the  female  showing  forty-eight  chromosomes,  of  which  four,  of  nearly  equal  size,  are  nearly  twice  the 
size  of  the  others.  This  agrees  precisely  with  the  relation  shown  in  a  double  group  of  Anasa  figured 
by  me  in  a  former  paper  ('06,  Fig.  2,  k)  which  shows  twice  the  normal  number  of  both  the  largest  and 
the  smallest  chromosomes. 

11  Since  published  in  Biol.  Bull.,  xiv,  5. 


88  Edmund  B.    Wilson 

ciated  to  form  a  definite  pentad  element  of  which  four  pass  to  one 
pole,  one  to  the  other,  while  the  remaining  fifteen  chromosomes 
divide  equally.  Half  the  spermatozoa  thus  receive  sixteen  chro- 
mosomes and  half  nineteen.  From  these  facts  it  is  clear  that  the 
sixteen-chromosome  class  must  be  male  producing,  the  nineteen- 
chromosome  class  female  producing,  according  to  the  formula: 

CALCULUS 

E§g  "  +  spermatozoon  »  -  3  =  zygote  n  -  3  (d") 
Egg  "_  +  spermatozoon  "          =  zygote  n  (  9) 


_ 


This  case,  together  with  that  of  Syromastes  (if  my  inference  regard- 
ing this  form  be  correct)  shows  that  we  must  considerably  enlarge 
our  previous  conceptions  as  to  the  relations  between  sex  produc- 
tion and  the  chromosomes;  for  we  can  no  longer  hold  that  only  a 
single  pair  are  involved.  In  Syromastes  there  are  two  such  pairs, 
in  Galgulus  several  pairs. 


It  is  evident  that  a  greater  variety  of  types  exists  in  regard  to 
the  sex  differences  than  was  indicated  in  the  brief  general  review 
given  in  the  third  of  my  "Studies"  (Wilson  '06.)  In  that  paper 
I  distinguished  three  types,  examples  of  which  are  given  by 
Protenor,  Lygaeus  and  Nezara;  but  the  number  must  now  be 
increased  to  at  least  five,  and  possibly  to  seven,  of  which  I  will 
now  give  a  brief  synopsis.  With  the  exception  of  Syromastes 
and  Diabrotica  this  synopsis  includes  only  species  of  which  both 
sexes  have  been  accurately  determined.  Forms  like  the  aphids, 
in  which  idiochromosomes  have  not  yet  been  positively  identified, 
have  been  omitted.  Seventeen  of  the  species  are  here  reported 
for  the  first  time  (one  or  both  sexes)  from  my  own  results  hitherto 
unpublished.  I  am  indebted  to  Dr.  Stevens  for  permission  to 
include  her  results  on  the  Diptera  and  on  Diabrotica,  which  are 
now  in  press  ('o8a,  'o8b). 


Studies  on   Chromosomes 


Both  sexes  with  the  same  number  of  chromosomes;  a  pair  of  equal  idiochro- 
somes  present  in  both.  No  visible  difference  between  the  two  classes  of  sperma- 
tozoa or  between  the  male  and  female  somatic  groups. 


FERTILIZATION  FORMULA 

Egg  n,  +  spermatozoon  r*  =  zygote  n  (  cT  or  9  ) 
Described  Case 


O 

d 

0 

0 

Specie 

Order 

F  amily 

| 

E 

15 

Authority 

J 

"b 

0 

Nezara  hilaris  Say 

Hemiptera  heteroptera 

Pentatomidae 

H 

'4 

Wilson   ('06) 

To  this  type  belongs  also  Oncopeltus  fasciatus  Dall,  one  of  the  Lygaeidae.  It 
is  further  probable  that  here  belong  many  forms  in  which  no  visible  sexual  differ- 
ences are  to  be  seen,  and  in  which  idiochromosomes  have  not  been  identified.  If  a 
particular  pair  of  chromosomes,  corresponding  to  idiochromosomes,  are  of  general 
occurrence,  though  not  visibly  distinguishable  from  the  others,  it  is  probable  that 
this  type  represents  the  most  frequent  condition  in  animals  generally. 

II 

Both  sexes,  and  both  classes  of  spermatozoa,  with  the  same  number  of  chromo- 
somes. The  male  with  a  pair  of  unequal  idiochromosomes,  half  the  spermatozoa 
receiving  the  large  one  and  half  the  small.  In  the  female  a  pair  of  equal  idiochromo- 
somes like  the  large  one  of  the  male. 

FERTILIZATION    FORMULA 

Egg  ^  (including  7)  +  spermatozoon  ^  (including  /')  =  zygote  n  (including  7/')  cT 
Egg  ™  (including 7)  +  spermatozoon^  (including 7)  =  zygote  n  (including 77)  9 


90 


Edmund  B.    Wilson 


Described  Cases 


Species 

Order 

Family 

c?  Somatic  No. 

i 

_o 

rt 

E 
£ 
o 

Authority     . 

Oebalus  pugnax  Fab. 

Hemiptera  heteroptera 

Pentatomidae 

10 

10 

Wilson 

Euschistus 

fissilis  Uhl. 

Hemiptera  heteroptera 

Pentatomidae 

H 

H 

Wilson  'O5b,  'o$c,  '06 

ictericus  L. 

Hemiptera  heteroptera 

Pentatomidae 

H 

»4 

Wilson  '06 

servus  Say 
tristigmus  Say 

Hemiptera  heteroptera 
Hemiptera  heteroptera 

Pentatomidae 
Pentatomidae 

'4 
H 

14 

H 

Wilson 
J  (^Montgomery  'o  i 
\  9  Wilson  '06 

variolarius  P.  B. 

Hemiptera  heteroptera 

Pentatomidae 

H 

H 

Wilson  '06 

Coenus  delius  Say 

Hemiptera  heteroptera 

Pentatomidae 

H 

H 

Wilson  'dfb,  '050,  '06 

Stiretrus  anchorago  Fab 

Hemiptera  heteroptera 

Pentatomidae 

H 

H 

Wilson 

Podisus 

maculiventris  Say   | 
(spinosus)            J 

Hemiptera  heteroptera 

Pentatomidae 

16 

16 

1    c?  Montgomery  '01 
^    9  Wilson  'o5b,  050,  '06 

Banasa 

dimidiata  Say 

Hemiptera  heteroptera 

Pentatomidae 

16 

16 

Wilson  '07  b 

calva  Say 

Hemiptera  heteroptera 

Pentatomidas 

26* 

26 

Wilson  'oyb 

Lygaeus 

turcicusFab. 

Hemiptera  heteroptera 

Pentatomidae 

H 

H 

Wilson  'o5b,  'o5c,  '06 

bicrucis  Say 

Hemiptera  heteroptera 

Pentatomidae 

H 

H 

Wilson 

Tenebrio  molitor 

Coleoptera 

Tenebrionidae 

20 

20 

Stevens  '05 

Chelymorpha  argus 

Coleoptera 

Chrysomelidae 

22 

22 

Stevens  '06 

Trirhabda  virgata 

Coleoptera 

Chrysomelidae 

28 

28 

Stevens  '06 

canadense 

Coleoptera 

Chrysomelidae 

3° 

30 

Stevens  '06 

Drosophila  ampelophila 

Diptera 

8 

8 

Stevens  '08  a 

Musca  domestica 

Diptera 

12 

12 

Stevens  '08  a 

Calliphora  vomitoria 

Diptera 

12 

12 

Stevens  '08  a 

Sarcophaga  sarraciniae 

Diptera 

12 

12 

Stevens  '08  a 

Scatophaga  pallida 

Diptera 

12 

12 

Stevens  '08  a 

Tetanocera  sparsa 

Diptera 

12 

12 

Stevens  '08  a 

Eristalis  tenax 

Diptera 

12 

12 

Stevens  '08  a 

*See  Type  Ha. 


Ill 


The  female  chromosome  groups  with  one  more  chromosome  than  the  male. 
Male  with  an  unpaired  idiochromosome  and  an  odd  spermatogonial  number,  half 
the  spermatozoa  receiving  the  idiochromosome  and  half  being  without  it.  Female 
with  an  equal  pair  of  idiochromosomes  like  the  unpaired  one  of  the  male. 

FERTILIZATION    FORMULA 

Egg  5  (including/)  +  spermatozoon  ™  —  i    =  zygote  n  —  I  (including/)  c? 
Egg  ™  (including/)  +  spermatozoon  T*  Concluding/)  =  zygote  n  (including //)  9 


Studies  on  Chromosomes 


91 


Described    Cases 


Species 

Order 

Family 

_o 

I 
•b 

rt 
I 

O 

Authority 

Largus  cinctus  H.  S.       ;  Hemiptera  heteroptera 

Pyrrochoridae 

ii 

12 

Wilson 

succinctus  L. 
Pyrrochoris  apterus  L. 

Hemiptera  heteroptera 
Hemiptera  heteroptera 

Pyrrochoridae 
Pyrrochoridae 

13 

14 

24 

Wilson 
/    9  Henking'gi 
\  c?  Wilson 

Alydus  pilosulus  H.  S. 
Harmostes                    \ 
reflexulus  Std.          J 

Protenor  belfragei  Hag. 

Hemiptera  heteroptera 
Hemiptera  heteroptera 

Hemiptera  heteroptera 

Coreidae 
Coreidae 

Coreidae 

13 
13 

13 

H 
14 

14 

Wilson  'o5b,  'o5c,  '06 
J    c?  Montgomery  '01 
1    9  Wilson  '06 
J    <?  Montgomery  'o  i 
\  9  Wilson  'osb,'o5c,'o6 

Leptocoris  trivittatus 

Say 

Hemiptera  heteroptera 

Coreidae 

13 

14 

Wilson 

Archimerus 

calcsrator  Fab. 

Hemiptera  heteroptera 

Coreidae 

is 

16 

Wilson 

Pachylis  gigas  Burm. 

Hemiptera  heteroptera 

Coreidae 

iS 

16 

Wilson 

Anasa  tristis  DeG. 
armigeraSay 

Hemiptera  heteroptera 
Hemiptera  heteroptera 

Coreidae 
Coreidae 

21* 
21 

22 
22 

Wilson  'o5b,  'o5c,  '06,  '073 
J  c?  Montgomery  '06 
\    9  Wilson 

sp. 

Hemiptera  heteroptera 

Coreidae 

21 

22 

Montgomery  '06 

Euthoctha  galestor 

Fab. 

Hemiptera  heteroptera 

Coreidae 

21 

22 

Wilson 

Leptoglossus  phyllopus 

L. 

Hemiptera  heteroptera 

Coreidae 

21 

22 

Wilson 

Margus 

inconspicuus  H.  S. 

Hemiptera  heteroptera 

Coreidae 

23 

24 

Wilson 

Chariesterus 

sntennator  Fab. 

Hemiptera  heteroptera 

Coreidae 

25 

26 

Wilson 

Corynocoris 

distinctus  Dall. 

Hemiptera  heteropters 

Coreidae 

25 

26 

Wilson 

Aprophora  quadrang- 

ularis 

Hemipters  homoptera 

Jassidae 

23 

24 

Stevens  '06 

Pceciloptera 

septentrionalis 

Hemipters  homoptera 

Fulgoridae 

27 

28 

Boring  '07 

pruinosa 

Hemiptera  homoptera 

Fulgoridae 

27 

28 

Bojing  '07 

Elater,  sp. 

Coleoptera 

Elateridae 

19 

20 

Stevens  '06 

Blatta  germanica 

Orthopters 

Blattidae 

23 

24 

/  d1  Stevens  '06 
\    9  Wsssilieff  '07 

Anaxjunius 

Odonsta 

Aeschindae 

27 

28 

LeFevre  and  McGill  '08 

*This  number,  disputed  by  Foot  and  Strobell  ('073,  b),  has  since  been  confirmed  by  my  own  reexami- 
nation  ('073,  '08)  and  by  thst  of  Lefevre  and  McGill  ('08)  and    others. 


Edmund  B.    Wilson 


IV 

Female  groups  (by  inference  only)  with  two  more  chromosomes  than  the  male. 
In  the  male  a  pair  of  unequal  idiochromosomes,  half  the  spermatozoa  receiving 
both  these  chromosomes,  and  hence  two  more  than  the  other  half.  In  the  female 
(by  inference  only)  two  such  pairs. 

FERTILIZATION  FORMULA 

Egg  ™  (including/,/)  +  spermatozoon^  —  2  =  zygote  n  —  2  (including  I, /)  d" 

Egg  ™  (including  I,  /')  +  spermatozoon  **   including  /,  /')  =  zygote  n    (including  I,  I,  i,  /,)    $    (by 
inference  only) 

Described  Case 


6 

6 

_o 

o 

Specie 

Order 

Family 

i 

o 

CO 

| 

Authority 

•b 

O 

Syromastes 
marginatus  L 

Hemiptera  heteroptera 

Coreidae 

22 

24 

\   ?  Wilson  (inferred) 

Female  groups  with  three  more  chromosomes  than  the  male.     Half  the  sperma- 
tozoa receiving  three  more  chromosomes  than  the  other  half. 

Egg  ™  +  spermatozoon^:  —  3  =  zygote  n  —  3  (c?) 
Egg  ^  +  spermatozoon  ™  =  zygote  n  (  9) 


Described  Case 


u 

0 
M 

Specie 

Order 

Family 

a 

s 

o 

CO 

rt 

G 
1 

Authority 

•b 

o 

Galgulus  oculatus  Fab. 

Hemiptera   heteroptera 

Galgulidae 

35 

38 

Payne  '08 

At  least  two  of  the  foregoing  types  may  be  complicated  by  the  presence  of  certain 
additional  chromosomes,  present  in  some  individuals  but  not  in  others  of  the  same 
species,  to  which  I  have  applied  the  name  of  "supernumerary  chromosomes."1 
The  number  of  these  varies  from  one  to  six  in  different  individuals  but  is  constant 
in  the  same  individual.     In  some  forms  (Metapodius,  Banasa)  these  supernumer- 

12  Wilson  'oyb,  'oyc.     A  detailed  description  is  now  in  preparation. 


Studies  on   Chromosomes 


93 


aries  accompany  a  typical  pair  of  unequal  idiochromosomes  (as  in  Type  II).  In 
other  forms  (Diabrotica),  the  supernumeraries  accompany  an  unpaired  idiochromo- 
some  (as  in  Type  III).  In  these  cases  definite  numerical  formulas  cannot  be 
given,  since  the  distribution  of  the  supernumeraries  is  variable  and  both  sexes  show 
a  variable  number  of  chromosomes  in  consequence  (directly  known  only  in  Meta- 
podius.)  For  the  present  these  cases  may  most  conveniently  be  treated  as  sub-types 
as  follows: 

Ila 

Forms  that  agree  with  Type  II  except  that  certain  individuals  may  possess,  in 
addition  to  a  pair  of  idiochromosomes,  one  or  several  supernumerary  chromosomes. 
The  cases  described,  with  the  numbers  of  chromosomes  observed,  are  as  follows: 


Species 

Order 

Family 

Somatic 
No. 

9 
Somatic                 Authority 
No. 

Banasa  calva 

Hemiptera 

Pentatomidae 

26             26           Wilson  'oyb 

heteroptera 

[26+,] 

Metapodius  terminalis 

Hemiptera 

Coreidae 

21*           22           Wilson  'O7b,  '08 

heteroptera 

22 

22+1 

22  +  1 

2-  +  2 

22  +  2 

22  +  3 

22  +  3 

22  +  4 

femoratus 

Hemiptera 

Coreidae 

22 

22+1 

Wilson  'ojb,  '08 

heteroptera 

22  +  2 

22  +  2 

22  +  3 

22  +  4 

22  +  4 

22  +  6 

granulosus 

Hemiptera 

Coreidae 

22 

22  +  3 

Wilson  'oyb,  '08 

heteroptera 

22+1 

22  +  4 

22  +  2 

22  +  4 

22  +  5 

*  This  number  occurs  only  in  Montgomery's    ('06)  material  of  this  species,  identification  of  which 
though  probably  correct,  is  not  absolutely  certain.    This  case  will  be  considered  in  a  later  publication. 

Ilia 

Forms  that  agree  with  Type  III  except  that  certain  individuals  may  possess,  in 
addition  to  an  unpaired  idiochromosome,  one  or  several  supernumerary  chromo- 
somes. Described  cases  as  follows: 


94 


Edmund  B.    Wilson 


c? 

9 

Specie 

Order 

Family 

Somatic 

Somatic 

Authority 

No. 

No. 

Diabrotica  12-punctata 

Coleoptera 

Chrysomelidae 

'9 

Stevens  '07,  '08 

soror 

19+1 

'9+3 

19+4 

Despite  the  apparent  diversity  of  the  types  that  have  been  enu- 
merated all  conform  to  the  common  principle  that  the  spermatozoa 
are  of  two  classes,  equal  in  number,  that  are  respectively  male 
producing  and  female  producing.  In  the  case  of  Type  I  this  is  no 
more  than  an  inference,  since  the  two  classes  cannot  be  distin- 
guished by  the  eye;  but  its  great  probability  will  be  admitted  in 
the  fact  that  the  forms  with  equal  idiochromosomes  are  connected 
by  forms  (such  as  Mineus)  in  which  only  a  slight  inequality  exists, 
with  those  in  which  the  inequality  is  very  marked  (Wilson 
'053).  The  facts  now  show  that  the  difference  between  the  two 
classes  of  spermatozoa  is  not  always  confined  to  a  single  pair 
of  chromosomes,  but  may  affect  two  pairs  (Syromastes)  or  even 
a  larger  number  (Galgulus).  It  is  noteworthy  that  in  every  case 
where  a  quantitative  difference  of  chromatin  exists  between  the 
sexes  it  is  always  in  favor  of  the  female,  whether  it  appear  in  a 
larger  number  of  chromosomes  or  in  the  greater  size  of  one  of  them. 
But  I  must  again  emphasize  the  fact  that  this  quantitative  differ- 
ence cannot  be  considered  as  the  primary  factor  that  differenti- 
ates the  two  classes,  for  in  the  first  class  such  a  difference  does  not 
exist,13  while  in  Metapodius,  even  in  the  same  species,  it  is  some- 

13  I  based  this  type  on  the  facts  observed  in  Nezara,  where  the  idiochromosomes  are  equal  in  size  in 
both  sexes.  This  is  not  in  accordance  with  the  later  observations  of  Montgomery  ('06)  who  believes 
that  in  the  Hemiptera  generally  the  two  components  (paternal  and  maternal)  of  every  chromosome 
pair  are  at  least  slightly  unequal — though  he  finds  the  idiochromosomes  of  Oncopeltus  equal  as  I  have 
also  since  observed.  A  reexamination  of  Nezara  confirms  my  original  account  of  this  form,  though  in 
some  individuals  the  idiochromosomes  often  appear  very  slightly  unequal.  A  careful  examination 
of  the  other  chromosomes,  particularly  the  small  m-chromosomes  (which  are  most  favorable  for  the 
purpose)  in  Alydus,  Anasa,  Archimerus,  Pachylis,  and  other  genera,  leads  me  to  a  very  skeptical  view  of 
Montgomery's  general  conclusion  on  this  point.  It  is  true  that  the  two  members  of  each  pair  vary 
slightly  in  relative  size,  and  are  not  always  exactly  equal;  but,  in  my  material  at  least,  it  is  clear  that 


Studies  on  Chromosomes  95 

times  the  female,  sometimes  the  male,  that  has  the  larger  number 
and  quantity.  I  therefore  adhere  to  the  view  that  if  the  primary 
and  essential  difference  between  the  two  classes  of  spermatozoa 
inhere  in  the  chromosomes  (there  is  of  course  room  for  difference 
of  opinion  on  this  point)  it  must  be,  or  originally  have  been, 
qualitative  in  nature. 

Since  the  appearance  of  my  third  "Study,"  in  which  some  general 
discussion  of  the  sex  chromosomes  was  offered,  there  has  appeared 
an  important  paper  by  Correns  ('07)  on  the  higher  plants,  the 
results  of  which,  as  he  points  out,  harmonize  remarkably  with 
those  based  on  the  cytological  evidence.  The  most  important 
of  his  results  is  the  experimental  proof  obtained  by  hybridizing 
experiments  on  Bryonia,  that  in  the  dioecious  species  the  pollen 
grains  are  male  producing  and  female  producing  in  equal  num- 
bers, quite  in  accordance  with  the  view  put  forward  by  McClung 
('02)  in  regard  to  the  spermatozoa  of  insects  and  proved  to  be 
correct  in  principle  by  the  work  of  Stevens  and  myself.  That  the 
same  result  should  appear  from  investigations  carried  out  on  such 
different  material  and  by  such  different  methods  certainly  gives 
good  ground  for  the  belief  that  as  far  as  the  male  is  concerned 
the  phenomenon  is  at  least  a  very  general  one.  Professor  Correns 
points  out  in  some  detail  the  extraordinarily  close  parallel  between 
his  experimental  results  and  the  cytological  ones  of  Stevens  and 
myself;  but  the  interpretation  that  he  offers  differs  materially 
from  both  those  that  I  suggested  in  an  analysis  of  my  observa- 
tions (Wilson  '06).  According  to  my  first  interpretation  (Castle's) 
both  sexes  are  assumed  to  be  sex  hybrids  or  heterozygotes.  The 
conclusion  of  Correns  is  that,  in  respect  to  the  active  sexual  ten- 
dencies of  the  gametes  that  produce  them,  only  the  male  is  a  sex 
hybrid  or  heterozygote  (d1  (?)),  while  the  female  is  a  homozy- 
gote  (9  9).14  This  interpretation  explains  the  numerical  equality 

this  is  merely  a  casual  fluctuation,  the  general  rule  being  equality.  This  variation  appears  in  dif- 
ferent cells  of  the  same  cyst  (as  may  be  seen  with  especial  clearness  in  the  m-chromosomes  in  side 
views  of  the  second  division  where  errors  due  to  foreshortening  may  be  eliminated).  It  would  be 
indeed  strange  if  these  relations  were  subject  to  no  variation  whatever. 

14  It  is  necessary  to  an  understanding  of  Correns's  view  to  bear  in  mind  that  the  gametes  are  not 
considered  to  be  "pure"  in  the  original  Mendelian  sense,  but  to  bear  both  sexual  possibilities,  one  of 
which  is  "active,"  the  other  "latent." 


96  Edmund  B.   Wilson 

of  the  sexes  in  accordance  with  the  Mendelian  principle  without 
the  necessity  for  assuming  selective  fertilization.  It  is  so  simple, 
and  seems  to  be  so  clearly  demonstrated  in  the  case  of  Bryonia, 
that  its  application  to  the  interpretation  of  sex  production  in 
general  is  very  tempting.  Correns  himself  believes  it  "very  prob- 
able" that  his  conclusion  will  apply  to  all  the  dioecious  flowering 
plants,  and  possible  that  it  may  also  hold  true  of  animals  (op.  cit., 
pp.  65,  66).  It  is  evident  that  in  their  superficial  aspects  the  cyto- 
logical  results  seem  to  bear  this  out.  Wherever  the  sexes  show 
visible  differences  in  the  somatic  chromosome  groups  the  female 
groups  consist  of  two  series  in  duplicate,  while  the  male  groups 
show  two  series  that  are  not  duplicates,  only  one  of  them  being 
identical  with  one  of  the  female  series.  As  far  as  the  chromosomes 
are  concerned,  and  from  a  purely  morphological  point  of  view, 
the  female  is  therefore  in  fact  a  homozygote,  the  male  a  hetero- 
zygote,  in  these  animals.  But  when  more  closely  scrutinized 
from  this  standpoint  the  interpretation  seems  by  no  means  so 
cleai  As  I  showed  in  my  third  "Study"  the  odd  chromosome 
of  the  male  must  be  derived  from  the  egg;  and  if  this  chromosome 
bears  the  sexual  tendency,  it  must  under  Correns's  hypothesis 
carry  the  female  tendency — which  is  a  reductio  ad  absurdum, 
since  it  is  not  accompanied  by  a  male-bearing  mate  or  partner  in 
the  male.  I  think  this  brings  clearly  into  view  the  following  alter- 
native. Either  the  females  of  these  insects  must  be  physiolog- 
ically heterozygotes  (as  I  assumed),  or  the  so-called  "sex  chromo- 
somes" (idiochromosomes)  do  not  bear  the  sexual  tendencies  but 
only  accompany  them  in  a  definite  way.  Which  of  these  possi- 
bilities is  the  true  one  may  be  left  to  further  research  to  decide. 
I  will  only  point  out  that  Professor  Correns  carefully  considers  the 
difficulties  that  his  interpretation  encounters  in  some  other  direc- 
tions, and  admits  that  it  must  be  modified  in  certain  cases — for 
example  in  the  honey  bee  and  in  Dinophilus,  in  which  latter  case 
he  too  is  compelled  to  admit  the  possibility  of  selective  fertiliza- 
tion. The  parthenogenetic  females  of  such  forms  as  the  aphids 
and  phylloxerans,  which  produce  both  males  and  females  without 
fertilization,  are  still  considered  by  Correns  as  homozygotes,  the 
production  of  males  being  assumed  to  be  determined,  if  I  under- 


Studies  on  Chromosomes  97 

stand  his  conception,  by  the  activation  of  the  "latent"  (not  to  be 
confused  with  the  "  recessive")  male  possibility  in  the  male  pro- 
ducing eggs.  This  is  doubtless  an  admissible  assumption,  though 
it  seems  to  me  to  put  a  considerable  strain  upon  the  general 
hypothesis.  The  more  natural  view  would  seem  to  be  the  one 
directly  suggested  by  the  facts,  i.e.,  that  the  parthenogenetic  stem- 
mother  aphid  is  a  heterozygote,  the  male  tendency  being  in  the 
condition  of  a  Mendelian  recessive.  But  I  will  not  enter  upon  a 
discussion  of  this  question,  which  is  now  in  a  condition  where  a 
little  observation  and  experiment  will  outweigh  a  large  amount  of 
hypothesis.  I  think,  however,  that  the  first  of  the  interpretations 
that  I  suggested  (following  Castle)  should  not  be  rejected  without 
further  data,  and  especially  not  until  the  question  of  selective 
fertilization  has  been  put  to  the  test  of  direct  experiment. 

Zoological  Laboratory 
Columbia  University 
February  13,  1908 

WORKS    REFERRED   TO 

BORING,  ALICE  M.  '07 — A  Study  of  the  Spermatogenesis  of  twenty-two  Species  of 

the  Membracidae,  Jassidae,  Cercopidae  and  Fulgoridae.     Journ. 

Exp.  Zool.,  iv,  4. 
CORRENS,  C. '07 — Die   Bestimmung  und  Vererbung  des    Geschlechtes.     Berlin, 

1907.     Also  (in  abbreviated  form)  in  Arch.  f.  Rassen-  u.  Ges.- 

Biologie,  iv,  6. 
DUBLIN,  L.  I.  '05 — The  History  of  the  Germ-cells  in  Pedicellina  Americana.     Ann. 

N.  Y.  Acad.  Sci.,  xvi,  i. 
FOOT,  K.,  and  STROBELL,  E.  C.  '073 — The  "Accessory  Chromosome"  of   Anasa 

tristis.     Biol.  Bull.,  xii. 
*O7b — A  Study  of  Chromosomes  in  the  Spermatogenesis  of  Anasa  tristis. 

Am.  Journ.  Anat.,  vii,  2. 
GROSS,  J.  '04 — Die  Spermatogenese  von  Syromastes  marginatus.     Zool.  Jahrb., 

Anat.  u.  Ontog.,  xx. 

'07 — Die  Spermatogenese  von  Pyrrochoris  apterus.     Ibid.,  xxiii. 
HENKING,  H.  '91 — Ueber  Spermatogenese  und  deren  Beziehung  zur   Eientwick- 

lung  bei  Pyrrorhoris  apterus.     Zeitschr.  f.  Wiss.    Zool.,  li. 
'92 — Untersuchungen  iiber  die  ersten    Entwicklungs-vorgange    in    den 

Eiern  der  Insekten,  111.     Ibid,  liv,  i. 


98  Edmund  B.   Wilson 

LEFEVRE,  G.,  and  McGiLL,  C.  '08 — The  Chromosomes  of  Anasa  tristis  and  Anax. 

junius.     Am.  Journ.  Anat.,  viii,  4. 
MEVES,  F.  '03 — Ueber  "Richtungskorperbildung"  im  Hoden  von  Hymenopteren . 

Anat.  Anz.,  xxiv. 
'07 — Die  Spermatocytenteilungen  bei  der  Honigbiene,  etc.    Arch.  mik. 

Anat.,  Ixx,  3. 
MONTGOMERY,  T.  H.  'oo — The  Spermatogenesis  of  Peripatus,  etc.     Zool.  Jahrb., 

Anat.  u.  Ontog.,  xiv. 
'01 — A  Study  of  the  Chromosomes  of  the  Germ-cells  of  Metazoa.      Trans. 

Am.  Phil.  Soc.,  xx. 

'04 — Some  Observations  and  Considerations  upon  the  Maturation  Phe- 
nomena of  the  Germ-cells.      Biol.  Bull.,  vi,  3. 
'06 — Chromosomes  in  the  Spermatogenesis  of  the  HemipteraHeteroptera. 

Trans.  Am.  Phil.  Soc.,  xx. 
McCLUNG,   C.    E.    '02 — The   Acessory   Chromosome — Sex  Determinant  ?     Biol. 

Bull.   iii. 
OVERTON,  J.  B.  '05 — Ueber  Reduktionsteilung  in  den  Pollenmutterzellen  einiger 

Dikotylen.     Jahrb.  wiss.  Bot.,  xlii,  i. 
PAULMIER,  F.  C.  '99 — The  Spermatogenesis  of  Anasa  tristis.     Journ.  Morph., 

Supplement. 
PAYNE,  F.  '08 — On  the  Sexual  Differences  of  the  Chromosome  groups  in  Galgulus 

oculatus.     Biol.  Bull.,  xiv,  5. 
SARGANT,  ETHEL  '96 — The  Formation  of  the  Sexual  Nuclei  in  Lilium  martagon. 

I,  Oogenesis.     Ann.  Bot.,  x. 
SCHREINER,  K.  E.  and  A.  '06 — Neue  Studien    iiber  die   Chromatinreifung    der 

Geschlechtszellen  (Tomopteris) .     Arch.  Biol.,  xx. 
STEVENS,  N.  M.  '03 — On  the  Ovogenesis  and  Spermatogenesis  of  Sagitta  bipunctata. 

Zool.  Jahrb.,  Anat.  u.  Ontog.,  xviii. 

'05 — Studies  in  Spermatogenesis  with  Especial  Reference  to  the  "Acces- 
sory Chromosome."  Carnegie  Institution,  Washington,  Pub. 
no.  36. 

'06 — Studies  in  Spermatogenesis.     II.  A  Comparati ve  Study  of  the  Hetero- 

chromosomes  in  certain  Species  of  Coleoptera,  Hemiptera  and 

Lepidoptera,  with   Especial   Reference  to  Sex  Determination. 

Ibid.,  Pub.  36,  II. 

'o8a — A  Study  of  the  Germ-cells  of  Certain  Diptera,  etc.,  Journ.  Exp. 

Zool.,  v,  3. 

'o8b — The  Chromosomes  in  Diabrotica,  etc.     Ibid.,  v,  4. 

SUTTON,  W.  S.  '02 — On  the  Morphology  of  the  Chromosome  group  in  Brachystola 
magna.     Biol.  Bull.,  iv,  I. 


Studies  on  Chromosomes  99 

WALLACE,  L.  B.  '05 — The  Spermatogenesis  of  the  Spider.     Biol.  Bull.,  viii. 
WASSILIEFF,  A.  '07 — Die  Spermatogenese  von  Blatta  germanica.    Arch.  mik.  Anat., 

Ixx. 
WILSON,    E.    B.   '053 — Studies    on     Chromosomes.     I.    The    Behavior   of  the 

Idiochromosomes  in  Hemiptera.     Journ.  Exp.  Zob'l.,  ii. 
'o5b — The  Chromosomes  in   Relation  to  the  Determination  of  Sex  in 

Insects.     Science,  xx,  p.  500. 
'050 — Studies   on    Chromosomes.     II.  The   paired    Microchromosomes, 

Idiochromosomes  and  Heterotropic  Chromosomes  in  the  Hemip- 
tera.    Ibid.,  ii. 
'06 — Studies  on   Chromosomes.     III.  The  Sexual   Differences    of  the 

Chromosome  Groups  in  Hemiptera,  with  some  Considerations 

on  the  Determination  and  Inheritance  of  Sex.     Ibid.,  iii. 
'073 — The  Case  of  Anasa  tristis.     Science,  xxv,  p.  191. 
'c>7b— Note  on  the  Chromosome  groups  of  Metapodius  and   Banasa. 

Biol.  Bull. 
'070 — The  Supernumerary  Chromosomes  of  Metapodius.     Read  before 

the    May  Meeting    of  the   N.    Y.  Acad.  of     Sci.      Science, 

xxvi,  677. 
'08 — The  Accessory  Chromosome  of  Anasa  tristis.     Read  before  the  Am. 

Soc.  of  Zoologists,  December,  '07.     Science,  xxvii,  690. 


EXPLANATION  OF  PLATES 

All  of  the  figures  are  reproduced  directly  from  photographs  by  the  author,  without  retouching.  The 
originals  were  taken  with  a  Spencer  -fa  oil-immersion,  Zeiss  ocular  6,  which  gives  an  enlargement  of  1500 
diameters.  The  admirable  method  of  focusing  devised  by  Foot  and  Strobell  was  employed.  They 
are  reproduced  at  the  same  magnification. 

PLATE  I 

(Photos  i  to  5,  10  to  23,  Syromastes  marginatus;  6  to  10,  Metapodius  terminalis;  24  and  25,  Pyrro- 
choris  apterus). 

i  and  2.    Spermatogonial  groups  of  Syromastes;  copied  in  Text-fig,  i,  a,  b. 

3  to  5.  Polar  views,  first  maturation  metaphase;  w-chromosome  at  the  center,  idiochromosome- 
bivalent  ("accessory"  chromosome)  outside  the  ring  at  the  left. 

6  and  7.  Corresponding  views  of  Metapodius,  typical  condition  with  the  two  separate  idiochromo- 
somes  outside  the  ring  at  the  left. 

8  and  9.    The  same;  exceptional  condition,  with  the  idiochromosomes  (at  the  left)  in  contact. 

10.    Polar  metaphase,  second  division,  Syromastes. 

II  to  17.  Side  views  of  the  same  division.  The  duality  of  the  idiochromosome  appears  in  12,  16 
and  17. 

18  to  23.  Early  prophases  of  first  maturation  division,  Syromastes.  Each  of  these  shows  the 
separate  wz-chromosomes,  and  in  all  but  No.  20  the  chromosome  nucleolus  (idiochromosome  bivalent) 
also  appears. 

24  and  25.    Spermatogonial  metaphases  of  Pyrrochoris  (copied  in  Text-figs.  2,  a,  i). 


UDIES     ON     CHROMOSOMES 

Edmond    K.    VHlsor. 


PLATE   1 


' 


t 


- 


lit 


« 


.  •  c;  ^ 


The  Journal  of   Experimental  Zoology,   Vol.   VI,   No     1. 


\VII.SON,    I'HOTO 


X'.E.  A  defect  In  the  plate  (not  preaen 
one  chromosome  at  the  right  side  of  Eh< 
shows  it  correctly. 


PLATE  II 
Pyrrochoris  apterus 

26  to  31.  Spennatogonial  groups,  each  showing  twenty-three  chromosomes,  including  the  large 
unpaired  idiochromosome;  30,  31  illustrate  the  rare  case  in  which  the  latter  appears  double,  owing  to 
marked  sigmoid  curvature.  These  photos  are  copied  in  Text-figs.  2,  c,  d,  e,  j,  k  and  /,  respectively. 

32  and  33.  Post-phases  shortly  following  last  spermatogonial  division;  the  chromosomes  still  distinct, 
idiochromosome  recognizable  by  its  large  size  and  deeper  color. 

34  and  35.  Presynaptic  stages  following  the  last,  showing  "caterpillar''  stage  of  idiochromosome 
and  small  nucleoli.  In  the  last  two  the  shortening  has  begun. 

36  to  38.  Further  condensation  of  the  idiochromosome;  initial  stages  of  synizesis;  apparent  duality 
of  the  idiochromosome  in  two  of  the  cells. 

39  to  42.     Synizesis,  showing  various  forms  of  the  chromosome  nucleolus. 

43  and  44.     Early  post-synaptic  stages. 

45  and  46.    Polar  metaphases,  first  spermatocyte  division. 

47  to  49.    Side  views  of  second  division. 

50  and  51.    Polar  metaphases,  second  division. 


i  the  original  negative)    causes 
to  appear   OnVble.      Fir.lj 


PLATE   II 


hdmond    H.    \\nson. 


2S  2v> 


30  «l 


«::? 


*.  i,r 


The  Journal  of  Experimental  Zoology,  Vol.   VI,   No     I. 


WILSON.     I'HOTO. 


$3  > 


'  EBM,  8,  WILSON, 

COLUMBIA  UNIVERSITY,  NtW  YORK, 


STUDIES  ON  CHROMOSOMES 

V  THE  CHROMOSOMES  OF  METAPODIUS,  A  CONTRI- 
BUTION TO  THE  HYPOTHESIS  OF  THE  GENETIC 
CONTINUITY  OF  CHROMOSOMES 


By 
EDMUND    B.  WILSON 


REPRINTED     FROM 


THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY 

Volume  VI         No.  2 


BALTIMORE,  MD.,  U.  S.  A. 
WILLIAMS  &  WILKINS  COMPANY 


STUDIES  ON  CHROMOSOMES 

V  THE  CHROMOSOMES  OF  METAPODIUS.  A  CONTRI- 
BUTION TO  THE  HYPOTHESIS  OF  THE  GENETIC 
CONTINUITY  OF  CHROMOSOMES1 

BY 

EDMUND  B.  WILSON 
WITH  ONE  PLATE  AND  THIRTEEN  FIGURES  IN  THE  TEXT 

The  genus  Metapodius  (Acanthocephala),  one  of  the  coreid 
Hemiptera,  shows  a  very  exceptional  and  at  first  sight  puzzling 
relation  of  the  chromosome-groups  which  has  seemed  to  me  worthy 
of  attentive  study  by  reason  of  its  significance  for  the  hypothesis 
of  .the  "individuality"  or  genetic  continuity  of  the  chromosomes. 
The  most  conspicuous  departure  from  the  relations-  to  which  we 
have  become  accustomed  lies  in  the  fact  that  different  individuals 
of  the  same  species  often  possess  different  numbers  of  chromo- 
somes, though  the  number  in  each  individual  is  constant.  An 
even  more  surprising  fact  is  that  in  all  of  my  own  material  every 
male  individual  possesses  at  least  22  spermatogonial  chromosomes, 
including  a  pair  of  unequal  idiochromosomes  like  those  of  the 
Pentatomidae,  while  in  Montgomery's  material  of  M.  terminalis 
every  male  has  but  21  spermatogonial  chromosomes,  one  of  which 
is  a  typical  odd  or  "accessory"  chromosome  (unpaired  idiochro- 
mosome).2 

The  present  paper  presents  the  results  of  an  investigation  of 
these  relations  that  has  now  extended  over  nearly  four  years,  in 
the  course  of  which  serial  sections  of  more  than  sixty  individuals 

1  Part  of  the  cost  of  collecting  and  preparing  the  material  for  this  research  was  defrayed  from  a  grant 
of  $500  from  the  Carnegie  Institution  of  Washington,  made  in  1906.    I  am  indebted  to  Rev.  A.  H. 
Manee,  of  Southern  Pines,  N.  C.,  for  valuable  cooperation  in  the  collection  of  material,  and  to  Dr. 
Uhler,  Mr.  Heidemann,  Mr.  Van  Duzee,  and  Mr.  Barber  for  aid  in  its  identification. 

2  By  Professor  Montgomery's  courtesy  I  have  been  enabled  to  study  thoroughly  his  original  prep- 
arations and  to  satisfy  myself  of  the  correctness  of  his  account  (Montgomery  '06).    I  also  owe  to  him 
a  number  of  unsectioned  testes  of  the  same  type. 

THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY,  VOL.  vi,  NO.  2. 


148  Edmund  B.   Wilson 

have  been  carefully  studied.  These  individuals  belong  to  three 
well  marked  species— M.  terminalis  Dall.  and  M.  femoratus  Fab. 
from  the  Eastern  and  Southern  States,  M.  granules  us  Dall.  from 
the  Western — all  of  which  show  a  similar  numerical  variation.3 
My  first  material,  including  sections  of  two  testes  of  M.  terminalis 
(Nos.  i,  2)  from  the  Paulmier  collection,  long  remained  a  complete 
puzzle  and  led  me  to  the  suspicion  that  the  material  was  patho- 
logical. This  possibility  was  eliminated  by  the  study  of  additional 
material  of  the  same  type;  but  the  contradiction  with  Montgom- 
ery's results  on  the  same  species  suggested  that  his  specimens 
were  not  correctly  identified  (Wilson  'o/a).  Continued  study  at 
length  convinced  me  that  this  supposition  too  was  probably  un- 
founded. If  the  identification  was  correct,  as  I  now  believe  it 
was,  M.  terminalis  is  a  species  that  varies  not  only  in  respect  to 
the  individual  chromosome  number  but  also  in  respect  to  the  sex- 
chromosomes,  certain  individuals  having  an  unpaired  "accessory" 
chromosome,  while  others  have  an  unequal  pair  of  idiochromo- 
somes.  The  latter  condition  alone  has  thus  far  been  found  in 
M.  femoratus  and  M.  granulosus.  The  essential  facts,  and  the 
general  history  of  the  spermatogenesis,  are  otherwise  closely 
similar  in  the  three  species. 

The  range  of  variation  in  the  number  of  chromosomes  is  in 
M.  terminalis  from  21  to  26,  in  M.  femoratus  from  22  to  27  or  28, 
and  in  M.  granulosus  from  22  to  27,  the  particular  number  (or 
its  equivalent  in  the  reduced  groups)  being  a  characteristic  feature 
of  the  individual  in  which  it  occurs.  I  do  not  mean  to  assert  that 
there  is  absolutely  no  fluctuation  in  the  individual.  In  this  genus, 
as  in  others,  apparent  deviations  from  the  typical  number  fre- 
quently are  seen,  and  real  fluctuations  now  and  then  appear;  but 
the  latter  are  so  rare  that  they  may  practically  be  disregarded. 
That  the  number  may  be  regarded  as  an  individual  constant 
(subject  to  such  deviations  as  are  hereafter  explained  (p.  185)  is 
abundantly  demonstrated,  not  only  by  the  agreement  of  large 
numbers  of  cells  from  the  same  individual  but  perhaps  even  more 

3  A  complete  list  of  the  individuals  examined,  arranged  by  localities,  is  given  in  the  Appendix  at 
p.-2O2,  Each  individual  is  there  designated  by  a  number  by  which  it  is  referred  to  in  the  text  and 
description  of  figures. 


Studies  on  Chromosomes 


149 


convincingly  by  the  definite  correlation  of  the  spermatocyte- 
groups  with  those  of  the  spermatogonia  of  the  same  individual. 
This  is  shown  in  the  following  table,  which  summarizes  the  facts 
thus  far  observed.4 

SUMMARY 


Somatic  number 
(spermatogonia  or 
ovarian  cells) 

First  spermatocyte 
division 

terminalis 

femoratus 

granulosus 

J 

? 

J 

9 

d1 

9 

21  

II 

12 

'3 

14 
15 

16 

i? 

9 

3 
5 
3 

2 
I 
0 

o 

0 

4 

2 

2 
O 
0 
0 

o 

3 
o 

2 
O 

2 
O 
0 

o 
o 

2 

I 
O 

I 

0 

i 

o 
I 

2 

4 

i 

4 

i 

0 

o 
o 

0 
0 

I 

2 
0 
0 

22  

27 

24..  .  . 

2C.  .  . 

26  

(27)   •  • 

28   (Z7  ?) 

Distribution  in  the  whole  group 


Total  somatic  number 

Number  of  males 

Number  of  females 

Totals 

21  

22  

7 

1  1 

27.  .  . 

7 

1  1 

14. 

ii 

2C.  .  . 

j 

•j 

6 

26  

7 

10 

(27)... 

I 

o 

i 

28  (27  ?)  . 

o 

i 

i 

Total  .  . 

AT 

IQ 

62 

4  The  somatic  numbers  of  the  males  are  in  each  case  determined  from  the  dividing  spermatogonia. 
Those  of  the  female  are  from  dividing  cells  in  various  parts  of  the  ovary — mainly  from  the  region  just 
above  or  below  the  end-chamber — some  of  them  undoubtedly  folicle-cells,  others  probably  young  nutri- 
tive cells  or  obgonia.  The  chromosome-groups  from  different  regions  differ  considerably  in  size,  but 
otherwis  show  the  same  general  characters.  With  a  very  few  exceptions  the  number  of  chromosomes 
has  been  determined  by  the  count  of  several  groups  from  the  same  gonad,  in  many  cases  by  the  count  of 
a  very  large  number.  In  many  individuals  hundreds  of  perfectly  clear  equatorial  plates  may  be  seen 
and  the  evidence  is  entirely  demonstrative.  In  seven  of  the  males  (owing  to  lack  of  mitoses,  or  to  defec- 
tive fixation)  the  somatic  number  has  been  inferred  from  that  shown  in  the  spermatocyte  divisions,  or 
vice  versa;  but  with  a  single  exception  both  numbers  have  been  directly  observed  in  other  individuals  of 
the  same  type.  I  am  therefore  confident  that  the  numbers  are  substantially  correct  as  given.  In  case  of 
the  female,  only  the  somatic  numbers  can  be  given,  since  the  maturation-divisions  are  not  available  for 
study. 


150  Edmund  B.    Wilson 

The  material  of  terminalis  is  from  New  Jersey,  Pennsylvania, 
Ohio,  North  Carolina,  South  Carolina  and  Georgia;  that  of  femor- 
atus  from  the  three  states  last  named;  that  of  granulosus  from 
Arizona.  The  variation  of  number  is  independent  of  locality,  and 
individuals  of  the  same  species  showing  different  numbers  were 
often  taken  side  by  side  on  the  same  food  plants.  It  is  equally 
independent  of  sex,  as  the  table  at  once  shows.  I  am  unable  to 
find  any  constant  correlation  between  the  number  of  chromosomes 
and  any  other  visible  structural  characters  of  the  adult  animals. 

Such  an  astonishing  range  of  variation  in  the  chromosome  num- 
ber in  the  same  species  seems  at  first  sight  to  present  a  condition 
of  chaotic  confusion.  But,  as  I  shall  endeavor  to  show,  the  first 
impression  thus  created  disappears  upon  more  critical  examination. 
Detailed  study  of  the  facts  proves  that  the  variation  is  not  indis- 
criminate but  affects  only  a  particular  class  of  small  chromo- 
somes that  are  distinguishable  from  the  ordinary  ones  both  by 
size  and  by  certain  very  definite  peculiarities  of  behavior.  These 
chromosomes  are  absent  in  all  of  Montgomery's  material;  in  my 
own  they  are  sometimes  present,  sometimes  absent,  the  total  num- 
ber varying  accordingly.  The  chromosomes  in  question  are  the 
ones  which  in  earlier  papers  I  have  called  the  "supernumeraries."5 
In  behavior  they  show  an  unmistakable  similarity  to  the  idiochro- 
mosomes;  and  for  reasons  given  beyond  I  believe  them  to  be  noth- 
ing other  than  additional  small  idiochromosomes,  the  presence  of 
which  has  resulted  from  irregularities  of  distribution  of  the  idio- 
chromosomes in  preceding  generations.  The  relations  seen  in 
Montgomery's  material  form  the  converse  case,  the  small  idio- 
chromosome  having  disappeared  or  dropped  out.  I  shall  try  to 
show  that  both  cases  are  probably  due  to  the  same  initial  cause. 


5  Wilson  'oya,  'oyb.  I  first  discovered  this  phenomenon  in  the  pentatomid  species  Banasa  calva 
('05!))  describing  the  single  supernumerary  as  a  "heterotropic  chromosome."  Later  ('oya)  a  single 
supernumerary  was  found  in  certain  individuals  of  Metapodius  terminalis,  and  other  numerical  varia- 
tions in  this  species  and  in  femoratus  and  granulosus  were  briefly  recorded;  but  at  that  time  I  did  not 
yet  fully  understand  the  facts.  Banasa  calva  is  the  only  form  oustide  the  genus  Metapodius,  in  a  totalof 
more  than  seventy  species  of  Hemiptera  I  have  examined,  in  which  supernumerary  chromosomes  have 
been  found.  Miss  Stevens  ('o8b)  has  recently  found  in  the  coleopteran  genus  Diabrotica  a  condition 
that  is  in  some  respects  analogous  to  that  seen  in  Metapodius. 


Studies  on  Chromosomes  151 

A      GENERAL    DESCRIPTION 

Since  the  phenomena  as  a  whole  are  somewhat  complicated,  I 
have  thought  it  desirable  to  bring  the  most  essential  facts  together 
for  ready  comparison  in  a  preliminary  general  account  illustrated 
by  a  limited  number  of  selected  figures  (Figs.  I,  2).  The  funda- 
mental type  of  the  genus  is,  I  believe,  represented  by  individuals 
that  possess  22  chromosomes  in  the  somatic  groups  of  both  sexes, 
and  in  which  no  supernumeraries  are  present  (Fig.  i,  </-/).  Two 
of  the  chromosomes  are  a  pair  of  very  small  m-chro  mo  somes,  like 
those  of  other  coreids;  two  are  a  pair  of  idiochromosomes  consist- 
ing in  the  male  of  a  large  and  a  small  member,  in  the  female  of 
two  large  ones;  while  the  remaining  18  are  ordinary  chromosomes 
or  "autosomes."  These  chromosomes  have  in  the  spermato- 
genesis  the  same  general  history  as  in  other  Hemiptera  heteroptera. 
In  the  first  division  the  idiochromosomes  are  separate  univalents, 
their  position  being  typically  (but  not  invariably)  outside  a  ring 
formed  by  the  nine  larger  bivalents  within  which  lies  the  small 
m-chromosome  bivalent  (Fig.  I,  d,  Photo  2).  This  division 
accordingly  shows  12  separate  chromosomes  (one  more  than  the 
reduced  or  haploid  number.)  In  the  second  division,  as  des- 
cribed beyond,  they  are  always  united  to  form  a  dyad  or  bivalent, 
composed  of  two  unequal  halves,  and  the  number  of  separate 
chromosomes  is  II.  The  spermatogonial  groups  possess  22  chro- 
mosomes (Fig.  i,  e]  of  which  the  small  idiochromosome  may  often 
be  recognized  as  the  smallest  of  the  chromosomes  next  to  the 
m-chromosomes;  but  it  does  not  differ  sufficiently  in  size  from  the 
other  chromosomes  to  be  always  certainly  distinguishable.8  In 
the  growth  period  the  idiochromosomes,  as  usual,  have  the  form 
of  condensed  deeply-staining  chromosome-nucleoli,  while  the  other 
chromosomes  are  in  a  vague,  faintly  staining  condition.  They 
are  usually  in  contact  but  not  fused  (Fig.  I,  /,  Photo  25),  thus  form- 

6  In  considering  the  relative  size-relations  it  is  important  to  bear  in  mind  that  the  apparent  size,  as 
seen  in  polar  view,  varies  considerably  with  the  degree  of  polar  elongation.  Still  more  important  is 
the  fact  (which  I  have  emphasized  in  a  preceding  paper)  that  in  the  first  division  univalent  chromosomes 
always  appear  relatively  much  smaller  than  they  do  in  the  spermatogonia.  This  is  the  case  with  the 
idiochromosomes  and  the  supernumeraries,  which  are  always  readily  recognizable  in  the  spermatocyte- 
divisions,  but  are  often  difficult  to  distinguish  in  the  spermatogonia. 


152  Edmund  B.    Wilson 


EXPLANATION  OF  FIGURES 

FIG.  i 

About  one-fourth  of  the  figures  were  drawn  upon  enlarged  photographs  by  the  method  described 
in  a  preceding  paper  (Wilson  '09).  The  others  are  from  camera  lucida  drawings.  In  all  cases  the  form, 
size,  and  grouping  of  the  chromosomes  are  represented  as  accurately  as  possible.  The  form,  size,  and 
general  appearance  of  the  spindles  are  shown,  but  no  attempt  has  been  made  to  represent  the  exact  details 
of  the  fibrillae.  Figs.  I  and  2  are  enlarged  about  3300  diameters,  the  others  a  little  less  than  3000 
diameters. 

Lettering,  in  all  the  Figures 

I,  large  idiochromosome  or  odd  chromosome; ;,  small  idiochromosome;  m,  m-chromosome;  p,  plas- 
mosome;  s,  supernumerary  chromosome.  In  cases  where  s  and  /  are  both  present  and  of  equal  size  it  is 
impossible  to  distinguish  between  them.  In  such  cases  I  have  as  a  rule  designated  as  /  the  one  lying 
nearest  to  7;  but  this  is  quite  arbitrary.  It  should  be  noted  also  that  7  cannot  always  be  distinguished 
from  the  smaller  of  the  ordinary  bivalents. 


Studies  on  Chromosomes 


a 


m 

• 


FIG.  i 

M.  terminalis 

/j-c  (No.  3),  2i-chromosome  form;  a,  first  spermatocyte  metaphase;  b,  spermatogonial  metaphasc; 
C,  nucleus  from  the  growth  period. 

d-f  (No.  1 9),  22-chromosome  form,  stages  corresponding  to  above, 
g-/  (No.  20,  Photo  4),  23-chromosome  form,  one  large  supernumerary. 
j-l  (No.  43),  23-chromosome  form,  one  small  supernumerary. 


154  Edmund  B.    Wilson 

ing  a  very  characteristic  bipartite  body;  but  in  a  good  many  cases 
they  are  separate  (Fig.  6,  c,  d,  Photo  26).  A  large  and  very  dis- 
tinct plasmosome  is  also  present. 

Such  a  group  of  22  chromosomes  may  be  regarded  as  the  type 
of  which  all  the  other  forms  may  be  regarded  as  variants,  and 
probably  as  derivatives.  In  forms  having  more  than  22  chromo- 
somes the  increase  in  number  is  due  to  the  presence  of  from  one  to 
six  supernumeraries.  These  vary  in  number  and  size  in  different 
individuals,  but  both  are  constant  in  a  given  individual.  Their 
maximal  size  is  equal  to  that  of  the  small  idiochromosome  (in 
which  case  they  are  indistinguishable  from  the  latter);  such  forms 
will  be  called  "large  supernumaries. "  Their  minimal  size, 
("small  supernumeraries")  is  about  the  same  as  that  of  the  m- 
chromosomes;  but  from  the  latter  they  are  always  distinguishable, 
in  the  male,  by  a  quite  different  behavior  in  the  maturation  pro- 
cess. When  a  single  supernumerary  is  present  it  may  be  either 
large  or  small,  its  size  being  (with  slight  variation)  constant  in  the 
individual.  When  more  than  one  is  present  all  may  be  of  the 
same  size  (the  most  usual  condition)  or  they  may  be  of  different 
sizes,  the  relation  being  again  an  individual  constant.  Whatever 
their  number  or  size  their  behavior  is  essentially  the  same  as  that 
of  the  idiochromosomes.  In  the  growth-period  they  have  a  con- 
densed form  and  are  typically  united  with  the  idiochromosomes 
to  form  a  compound  chromosome-nucleolus,  the  components  of 
which  are  often  distinctly  recognizable  and  vary  in  number  with 
the  number  of  the  supernumeraries.  In  the  first  division  they 
divide  as  separate  univalents,  and  this  division  accordingly  shows 
as  many  chromosomes  above  12  as  there  are  supernumeraries— 
i.e.,  if  the  spermatogonial  number  be  22  +  n,  the  number  in  the 
first  division  is  typically  12  +  n.  Their  typical  position  in  this 
division  is,  like  that  of  the  idiochromosomes,  outside  the  ring  of 
larger  bivaients,  though  there  are  many  exceptions.  In  the  sec- 
ond division  they  are,  as  a  rule,  again  associated  with  the  idio- 
chromosomes to  form  a  compound  element,  though  not  infre- 
quently one  or  more  of  them  may  be  free  from  the  others. 

A  definite  correlation  thus  appears  in  each  individual  between 
the  number  and  relative  sizes  of  the  chromosomes   seen   in   the 


Studies  on  Chromosomes  155 

maturation-divisions  and  in  those  of  the  spermatogonia;  and  it 
also  appears  in  the  number  and  size  of  the  components  of  the 
chromosome-nucleoli  when  these  can  be  distinctly  recognized. 
Figs.  I  and  2  illustrate  this  correlation  and  epitomize  the  most 
essential  facts.  These  figures  have  been  selected  from  a  much 
larger  number  to  show  the  clearest  and  most  typical  conditions. 
Some  of  them  are  enlarged  from  the  photographs  reproduced  in 
Plate  I.  Many  others,  with  an  account  of  secondary  variations, 
are  given  beyond.  Each  horizontal  row  of  figures  represents 
three  stages  of  the  same  type  which,  with  two  exceptions,  are  all 
from  the  same  individual.  The  left  hand  figure  in  each  row  shows 
the  typical  arrangement  of  the  chromosomes  in  the  metaphase 
of  the  first  spermatocyte-division,  the  middle  figure  a  spermato- 
gonial  group,  and  the  right  hand  one  a  nucleus  from  the  growth 
period,  to  show  the  chromosome-nucleolus  together  with  some  of 
the  diffused  ordinary  chromosomes. 

Fig.  i,  a-c  (terminalis,  No.  3),  represent  these  three  stages  in 
an  individual  of  the  2i-chromosome  type  (Montgomery's  material) 
showing  II  chromosomes  in  the  first  division,  21  in  the  sper- 
matogonia, and  a  single  chromosome-nucleolus  in  the  growth 
period.  (Additional  figures  of  this  individual  in  Fig.  3.)  Fig.  I, 
d-f  (terminalis,  No.  19),  show  the  22-chromosome  type,  with 
a  small  idiochromosome  present  in  addition  to  the  large  one. 
The  small  idiochromosome  (/)  is  distinguishable  in  Fig.  I,  e. 
(Additional  figures  in  Figs.  4-6.) 

Fig.  I,  g-i  (terminalis,  No.  20),  show  the  23-chromosome  type, 
with  one  large  supernumerary.  In  the  spermatogonial  group  (h) 
this  chromosome  and  the  small  idiochromosome  are  probably  rep- 
resented by  the  two  designated  as  ;  and  s.  The  nucleus  from  the 
growth-period  (/'),  shows  the  plasmosome  (/>)  and  a  tripartite 
chromosome-nucleolus  formed  by  the  idiochromosomes  and  the 
supernumerary  attached  in  a  row  (cf.  Photo  27;  additional  figures 
in  Figs.  7-8).  Fig.  I,  /-/  (terminalis,  No.  43),  show  a  23-chro- 
mosome group  with  one  small  supernumerary.  This  clearly 
appears  in  the  spermatogonial  group  (/);  and  the  small  idiochro- 
mosome (/)  is  also  distinguishable.  In  the  nucleus  from  the 
growth-period  (/),  the  supernumerary  and  small  idiochromosome 


156  Edmund  B.    ffilson 

are  united  (/,  s)  the  large  idiochromosome  (/)  being  separate. 
(Additional  figures  in  Figs.  7,  8.) 

Fig.  2,  a-c  (terminalis,  No.  21),  show  the  corresponding  stages 
in  an  individual  of  the  24-chromosome  type,  with  two  large  super- 
numeraries. Their  identification  in  the  spermatogonial  group 
is  somewhat  doubtful.  (Additional  figures  in  Fig.  10.) 

Fig.  2,  d,  e  (terminalis  No.  34),  show  a  25-chromosome  type 
with  three  large  supernumeraries.  The  growth-period  (/)  is  from 
an  individual  of  granulosus  (No.  54)  that  is  possibly  of  the  26- 
chromosome  type.  (Additional  figures  in  Fig.  12.) 

Fig.  2,  g,  h  (femoratus  No.  42),  and  z  (granulosus,  No.  60) 
show  the  26-chromosome  type  with  four  large  supernumeraries. 
(See  Photo.  28,  additional  figures  in  Figs.  9,  10.) 

Fig.  2,  j—l  (femoratus,  No.  40),  are  from  a  very  interesting  indi- 
vidual of  the  26-chromosome  type,  with  two  large  and  two  small 
supernumeraries  (additional  figures  in  Figs.  9,  10).  The  sperma- 
togonia  of  this  individual  (k)  uniformly  show  26  chromosomes, 
including  four  very  small  ones  (two  m-chromosomes,  two  small 
supernumeraries),  but  the  large  supernumeraries  and  the  small 
idiochromosomes  are  doubtful.  No  case  was  found  in  which  all 
of  the  six  components  of  the  chromosome-nucleolus  could  be  seen; 
/  shows  five  of  them,  including  the  two  small  ones. 

B      ADDITIONAL    DESCRIPTIVE    DETAILS 

I  will  now  give  a  somewhat  more  detailed  and  critical  account 
of  the  facts.  Taken  as  a  whole,  the  series  (including  nearly  300 
slides  of  serial  sections)  presents  a  profusion  of  evidence  on  many 
cytological  questions  that  could  not  be  adequately  described  save 
in  a  large  monograph;  but  I  will  here  limit  the  account  mainly  to 
the  numerical  and  topographical  relations  of  the  chromosomes. 
The  clearness  of  the  preparations  is  such  that  nearly  all  the  prin- 
cipal phenomena  might  have  been  illustrated  by  photographs  (of 
which  upwards  of  200  have  been  prepared).  Thirty  of  these 
are  reproduced  in  Plate  I,  less  for  the  purpose  of  giving  the 
evidence  in  detail  than  of  illustrating  its  character  to  those  not 
directly  familiar  with  this  material. 


Studies  on  Chromosomes 


s 

a 


Aft 


d 


*  •* 

5 


FIG.  2 

et-e,  M.  terminalis;  /,  /,  granulosus;  g-h,  j-l,  femoratus. 
a-c  (No.  21),  24-chromosome  form,  two  large  supernumeraries. 
d-e  (No.  34),  25-chromosome  form,  three  large  supernumeraries. 
/  (No.  54),  growth-period,  25-  or  26-chromosome  form. 
g-h  (No.  42  Photo  8),  26-chromosome  form,  four  large  supernumeraries. 
»  (No.  60),  26-chromosome  form,  growth-period. 
j-l  (No.  40),  26-chromosome  form,  two  large  and  two  small  supernumeraries. 


158  Edmund  B.   Wilson 

I  Individuals  having  twenty-one  spermatogomal  Chromosomes, 
including  an  unpaired  I  dio  chromosome.  Small  Idiochromo- 
some  and  Supernumeraries  absent 

To  this  group  belong  only  the  specimens,  all  males,  collected  by 
Montgomery  at  West  Chester,  Pa.,  of  which  I  have  examined  nine 
individuals,  all  of  which  have  essentially  the  same  characters.7 
Montgomery  ('01)  originally  described  these  forms  as  having  22 
spermatogonial  chromosomes  but  subsequently  ('06)  corrected  this 
to  21,  describing  the  phenomena  as  agreeing  in  all  essential  respects 
with  those  seen  in  Anasa  and  other  coreids.  A  study  of  the  orig- 
inal preparations  has  enabled  me  to  confirm  this  later  account  in 
every  essential  point.  After  the  synizesis  or  contraction  phase  of 
synapsis  (as  in  all  individuals  of  the  genus)  the  ordinary  chromo- 
somes appear  in  the  form  of  rather  delicate  spireme-like  threads, 
longitudinally  split.  In  later  stages  of  the  growth-period  they 
shorten,  become  irregular,  lose  their  staining  capacity,  and  assume 
the  vague,  pale  condition  characteristic  of  so  many  other  forms. 
In  the  early  prophases  of  the  first  division  they  become  more  defi- 
nite, stain  more  deeply,  and  appear  as  coarse  longitudinally  split 
rods  that  often  show  an  indication  of  a  transverse  division  at  the 
middle  point,  or  in  the  form  of  the  double  crosses  as  described  by 
Paulmier  in  Anasa  ('99).  In  the  later  prophases  they  condense 
still  further  to  form  nine  compact  bivalents  which  finally  arrange 
themselves  in  a  more  or  less  regular  ring.  The  equatorial  plate 
of  the  first  division  always  shows  in  polar  view  n  chromo- 
somes (Fig.  3,  <a,  by  Photo  l).  In  the  most  typical  case  the  univa- 
lent  idiochromosome  lies  outside  this  ring,  but  it  sometimes  lies 
in  or  inside  it.  The  small  m-chromosome  bivalent  is  always  near 
the  center  of  the  ring.  In  side  view  the  larger  bivalents  are  either 
dumb-bell  shaped  or  more  or  less  distinctly  quadripartite,  in  the 

7  These  were  taken  from  magnolia  trees.  In  the  summer  of  1907  I  collected  in  the  same  locality 
two  males  and  three  females,  all  from  blackberry  bushes.  To  my  disappointment,  these  differ  from 
Montgomery's  specimens,  one  male  having  22  spermatogonial  chromosomes,  the  other  23;  while  the 
ovarian  cells  have  in  one  female  23  and  in  the  other  two  24  chromosomes.  It  is  possible  that  a  different 
species  fell  into  Montgomery's  hands,  perhaps  an  introduced  form;  but  both  the  structure  of  the  testis 
and  the  character  of  the  chromosome-groups  agree  so  exactly  with  my  own  material  that  I  now  believe 
that  Montgomery's  identification  was  probably  correct. 


Studies  on  Chromosomes  159 

latter  case  appearing  dumb-bell  shaped  as  seen  in  polar  view. 
The  eccentric  idiochromosome  is  of  nearly  the  same  size  as  the 
smallest  of  the  large  bivalents  and  is  often  indistinguishable  from 
the  latter  except  by  its  position.  All  these  chromosomes  divide 
equally  in  this  division,  the  m-chromosomes  usually  leading  the 
way  in  the  march  towards  the  poles,  while  the  idiochromosomes 
often  lag  slightly  behind  the  others. 

The  second  division  likewise  shows  II  chromosomes  in  polar 
view  (3,  c,  d] ;  but  the  regular  grouping  characteristic  of  the  first 
division  is  now  usually  lost,  the  ring  formation  being  often  no 
longer  apparent,  while  either  the  m-chromosome  or  the  idiochro- 
mosome may  now  occupy  any  position.8  In  this  mitosis  all  the 
chromosomes  divide  except  the  idiochromosome  which  lags  behind 
the  others  and  finally  passes  undivided  to  one  pole  (Fig.  3,  e-h, 
Photos  14,  15)  as  Montgomery  described.  The  nucleus  formed 
at  this  pole  thus  receives  II  chromosomes,  the  sister  nucleus  but 
10,  precisely  as  in  Anasa,  Narnia,  Chelinidea  or  Leptoglossus. 
This  is  proved  beyond  all  doubt  by  polar  views  of  the  anaphases, 
showing  the  sister  groups  lying  one  above  the  other  in  the  same 
section  (Fig.  3,  h).  In  the  particular  example  figured  the  idio- 
chromosome lies  eccentrically,  but  this  is  quite  inconstant. 

The  spermatogonia  (Fig.  3,  /,  /)  always  show  21  chromosomes, 
a  largest  and  a  smallest  pair  being  always  distinguishable.  The 
unpaired  idiochromosome  cannot  be  distinguished  from  the  others. 
The  m-chromosomes  are  usually  equal,  but  sometimes  appear 
slightly  unequal. 

In  the  growth-period  the  m-chromosomes  and  the  idiochromo- 
some have  the  same  history  as  in  other  coreids.  The  former  are 

8  The  regrouping  of  the  chromosomes  in  the  second  division,  first  described  by  Paulmier  ('99)  in 
Anasa  tristis,  is  characteristic  of  the  Coreidae  generally,  an  eccentric  position  of  the  idiochromosome 
being  a  nearly  constant  feature  of  the  first  division  but  not  of  the  second.  Failure  to  recognize  this 
fact  in  the  case  of  Anasa  tristis  seems  to  have  been  one  of  the  main  sources  of  error  in  the  entirely  mis- 
taken conclusions  of  Foot  and  Strobell  ('oya,  'oyb)  regarding  this  species.  (Cf.  Lefevre  and  McGill,  '08.) 
Demonstrative  evidence  on  this  point  is  given  by  polar  views  of  rather  late  anaphases  in  which  every 
chromosome  of  each  daughter  plate  may  be  seen  in  the  same  section.  Such  views,  of  which  I  have 
studied  many,  both  in  Anasa  and  in  other  genera,  show  that  oneof  the  chromosomes  may  indeed  occupy 
an  eccentric  position,  and  may  there  divide;  but  in  such  cases  the  odd  chromosome  is  always  found 
elsewhere  in  the  group,  lying  either  in  or  near  one  of  the  daughter-groups  and  not  in  the  other.  When 
the  odd  chromosome  is  eccentric  it  is  found  in  one  of  the  daughter  groups  but  not  in  the  other. 


i6o 


Edmund  B.   Wilson 


typically  separate,  and  at  first  diffuse  (as  in  Anasa  or  Alydus). 
Later  they  condense  to  from  two  spheroidal  bodies  that  conjugate 
in  the  late  prophase  to  form  the  central  small  bivalent  and  are 
almost  immediately  separated  again  by  the  division.  The  idio- 


,% 

% 


a 


d 


h 


FIG.  3 

M.  terminalis  (Montgomery's  material  (Nos.  3-11),  2i-chromosome  form) 

a,  b,  first  division,  polar  view  (Photo  i);  c-d,  second  division;  e,  f,  g,  side  views  of  second  division 
(Photos  14,  15);  h,  sister-groups  from  the  same  spindle,  in  one  section,  anaphase  second  division,  one 
showing  10  chromosomes  the  other  1 1. 

/'-_/,  spermatogonial  groups,  21  chromosomes;  k-l,  early  and  late  growth-period. 

chromosome  has  throughout  the  early  and  middle  growth-period 
the  form  of  a  single  spheroidal  or  ovoidal  intensely  staining  chro- 
mosome-nucleolus,  which  shows  in  brilliant  contrast  to  the  other 
chromosomes  (Fig.  3,  k,  /,  Photo  24).  This  body  is  sometimes 
slightly  constricted  in  the  earlier  period.  Later  it  is  always  con- 


Studies  on  Chromosomes  161 

stricted,  assuming  the  bipartite  form  in  which  it  enters  the  equa- 
torial plate  to  form  the  eccentric  chromosome.  Throughout  the 
growth-period  a  large  plasmosome  is  also  present,  usually  separate 
from  the  chromosome-nucleolus.  In  properly  stained  sections 
these  two  bodies  differ  so  markedly  in  staining  reactions  that  they 
cannot  for  a  moment  be  confused.  In  haematoxylin  preparations 
the  chromosome-nucleus  is  intensely  black,  the  plasmosome  pale 
yellowish,  bluish  or  gray.  In  Montgomery's  safranin-gentian 
preparations  (though  now  somewhat  faded)  the  former  is  bright 
red,  the  latter  bluish  or  nearly  colorless. 

There  are  no  females  in  Montgomery's  material;  but  in  view 
of  the  relations  known  in  many  other  related  forms  it  may  safely  be 
concluded  that  the  ii-chromosome  spermatozoa  are  female-pro- 
ducing, and  that  the  female  somatic  number  in  this  race  is  22. 

2  Individuals  with  twenty-two  Chromosomes  in  the  somatic  Groups 
of  both  Sexes  including  a  pair  of  unequal  1 dio chromosomes  in 
the  Male,  and  a  Pair  of  equal  large  ones  in  the  Female 

This  condition  has  been  found  in  seven  males  and  four  females, 
all  three  species  being  represented.  The  three  species  closely 
agree  in  all  the  phenomena. 

To  the  males  of  this  type  precisely  the  same  description  applies 
as  to  the  foregoing  case  except  that  a  small  idiochromosome  is 
present  in  addition  to  the  "odd"  or  "accessory"  chromosome. 
The  latter  is  now  indistinguishable  from  a  "large  idiochromosome, " 
and  the  identity  of  these  two  forms  of  chromosomes,  on  which  I 
have  laid  stress  in  former  papers,  is  thus  fully  demonstrated.  This 
appears  most  clearly  in  the  maturation  divisions.  In  the  first 
division  the  chromosomes  show  the  same  grouping  as  in  the  21- 
chromosome  forms,  but  a  small  idiochromosome  accompanies 
the  "accessory,"  frequently  lying  beside  it  outside  the  principal 
ring,  though  sometimes  being  in  or  inside  the  latter  (Fig.  4,  a-/, 
Photos  2,  3).  This  chromosome  is  always  recognizable  as  the 
smallest  of  all  the  chromosomes  except  the  m-chromosomes,  and  it 
is  in  general  about  half  the  size  of  the  large  idiochromosome  or 
slightly  less.  All  the  chromosomes  now  divide  equally  (Fig.  4,  /, 


162  Edmund  B.   Wilson 

Photo  11),  12  chromosomes  passing  to  each  pole.  The  second 
division  immediately  follows  without  the  intervention  of  a  "resting 
stage,"  and  the  chromosomes  undergo  the  same  regrouping  as 
that  described  for  the  2i-chromosome  forms.  As  this  takes  place, 
the  two  idiochromosomes  conjugate  to  form  an  unequal  bivalent 
(precisely  as  in  Lygaeus  or  Euschistus);  so  that  when  the  equato- 
rial plate  reforms  but  II  (instead  of  12)  chromosomes  appear  in 
polar  view  (Fig.  5,  a-c,  Photo  12).  The  idiochromosome-biva- 
lent  now  usually  lies  near  the  center  of  the  group  (contrasting 
with  the  first  division),  and  the  m-chromosome  is  usually  not  far 
from  it.  Such  views  are  almost  indistinguishable  from  those  of 
the  2i-chromosome  individuals,  since  the  small  idiochromosome 
is  covered  by  the  large  one  and  only  appears  in  side  view.  In  the 
course  of  the  division  the  idiochromosome  bivalent  separates  into 
its  two  components,  which  pass  to  opposite  poles,  while  all  the 
other  chromosomes  divide  equally.  The  idiochromosomes  at  first 
separate  more  rapidly  than  the  other  daughter-chromosomes  (Fig. 
5,  /,  /z),  as  in  other  genera,  but  as  the  division  proceeds  the  reverse 
condition  prevails,  so  that  the  two  idiochromosomes  are  seen  lag- 
ging on  the  spindle  between  the  diverging  daughter  groups  (Fig.  5 
/'-/).  In  the  later  stages  one  passes  to  each  pole.  There  is 
much  variation  in  this  process.  Often  the  two  move  at  the  same 
rate  so  that  in  the  late  anaphases  one  may  be  seen  entering  each 
pole  (Fig.  k,  /,  Photo  17).  Not  uncommonly,  however,  one  or 
the  other  lags  behind  upon  the  spindle  (usually  the  large  one, 
though  Fig.  5,  /,  shows  the  reverse  case)  giving  a  condition  that 
exactly  resembles  that  seen  in  the  2i-chromosome  forms  (Fig  5, 
m,  n),  but  earlier  anaphases  in  the  same  cysts  at  once  show  the 
difference.  It  is  no  less  conclusively  shown  by  polar  views  of  the 
late  anaphases,  in  which  each  daughter-group  is  seen  to  consist 
of  1 1  chromosomes,  ten  of  which  are  duplicated  in  the  two  while  the 
the  eleventh  is  in  one  case  the  large,  in  the  other  the  small  idio- 
chromosome (Fig.  5,  q,  r,  s,  t). 

The  difference  between  the  two  types  is  shown  with  almost  equal 
clearness  by  the  chromosome-nucleoli  of  the  growth-period.  In 
the  2i-chromosome  type,  as  already  stated,  this  body  is  single.  A 
similar  appearance  is  sometimes  given  in  the  22-chromosome  indi- 


Studi 


ies  on 


Ch 


romosomes 


163 


•t 


•  »••' 


« 


f 


t> 


/       «? 


FIG.  4 

22-chromosome  forms 

/j-/,  first  division;  a,  t,  term.  No.  19,  typical  (Photo  2);  c,  term.  No.  12  (Photo  3);  <f,  e,  fem.  No.  29 
/.  term.  No.  12;  £,  A,  term.  No.  19,  idiochromosomes  united;  /',  fem.  No.  29,  same  condition;  j,  gran.  No. 
47;  k,  fem.  No.  29,  first  division,  side  view,  idiochromosomes  united;  /,  fem.  No.  46  (Photo  1 1),  first  divi- 
sion, anaphase,  division  of  both  idiochromosomes. 

m-q,  spermatogonial  groups;  m,  term.  No.  19;  n,  o,  fem.  No.  46;  p,  q,  fem.  No.  29. 

r-t,  ovarian  groups;  r,  term.  No.  24:  s,  term.   No.  44;  t,  term.  No.  23,  exceptional  form  and  grouping. 


164  Edmund  B.    Wilson 

viduals,  owing  to  close  union  of  the  two  idiochromosomes.  But 
in  very  many  cells  of  this  period  the  chromosome-nucleolus  con- 
sists of  two  very  distinct  unequal  moieties,  in  contact  (Fig.  6,  a, 
/>,  Photo  25),  or  not  infrequently  widely  separated  (Fig.  6,  c,  d. 
Photo  26).  When  in  contact  they  form  a  double  body  closely 
similar  to  the  idiochromosome-bivalent  of  the  second  division. 
There  can  be  no  question  of  confusing  either  of  these  bodies  with 
the  plasmosome,  since  the  latter,  showing  its  characteristic  stain- 
ing reactions,  is  also  present. 

In  the  late  prophases  of  the  first  division  the  idiochromosomes, 
if  previously  united,  almost  invariably  part  company  to  divide  as 
separate  univalents,  as  in  other  Hemiptera;  but  they  usually 
remain  near  together  outside  the  principal  ring.  Only  very  excep- 
tionally do  they  divide  together. 

The  spermatogonial  groups  (Fig.  4,  m-q)  uniformly  show  22 
chromosomes,  and  in  some  cases  the  small  idiochromosome  may 
be  recognized  by  its  small  size  (m,  q).  This  is,  however,  not 
nearly  so  marked  as  in  the  first  division,  since  it  now  appears  rela- 
tively twice  as  large,  owing  to  the  univalent  character  of  the  other 
chromosomes,  and  often  it  cannot  certainly  be  distinguished  from 
the  smaller  of  these  (n,  p). 

These  facts  make  it  clear  that  if  the  small  idiochromosome  be 
supposed  to  disappear,  the  entire  series  of  phenomena  would  be- 
come identical  with  those  shown  in  the  2i-chromosome  individuals, 
the  large  idiochromosome  now  appearing  as  the  odd  or  "acces- 
sory" chromosome. 

The  unreduced  female  groups  of  this  type  (ovarian  cells)  are 
closely  similar  to  those  of  the  male  (Fig.  4,  r—t)  but  a  small  idio- 
chromosome can  never  be  distinguished.  The  absence  of  this 
chromosome  cannot  be  so  convincingly  shown  in  Metapodius  as 
in  such  forms  as  Lygaeus  or  Euschistus,  owing  to  its  greater  rela- 
tive size.  Nevertheless,  after  the  detailed  study  of  many  female 
groups  I  am  convinced  that  this  chromosome  is  not  present,  and 
that  all  the  chromosomes  may  be  equally  paired.  Apart  from 
analogy,  therefore,  I  think  the  conclusion  reasonably  safe  that 
in  Metapodius,  as  in  other  forms,  the  unequal  idiochromosome- 
pair  of  the  male  is  represented  in  the  female  by  a  large  equal  pair, 


Studies  on  Chromosomes 


:•:••.*:.••:*  A 

'  -  •  •'*  '  •  <  € 


urn. 

w. 


FIG.  5 

22-chromosome  forms 

a-c,  second  division,  polar  view;  a,  fern.  No.  19;  b,  fern.  No.  28;  c,  gran.,  47  (Photo  12). 

d-p,  second  division,  side  view;  d-h,  fern.  No.  29,  metaphases,  separation  of  idiochromosomes;  i  j, 
term.  No.  19,  anaphases,  lagging  of  one  idiochromosome;  k-m,  gran.,  No.  47,  late  anaphases  (Photo  17); 
w,  term.,  No.  19,  late  anaphase,  lagging  large  idiochromosome;  o,  fern.,  No.  46,  exceptional  condi- 
tion, both  idiochromosomes  passing  to  one  pole  (Photo  18);  p,  term.  No.  19,  similar  form;  q,  r,  term., 
No.  19,  sister  anaphase  groups,  from  the  same  spindle;  3,  t,  fern.,  No.  29,  the  same. 


1 66 


Edmund  B.    Wilson 


and  that,  accordingly,  the  usual  rule  holds  in  regard  to  fertiliza- 
tion. 

Exceptional  conditions.  There  are  two  conditions,  rarely  seen, 
that  are  of  interest  for  comparisons  with  other  species.  Now  and 
then  the  idiochromosomes  fail  to  separate  for  the  first  division, 
but  remain  in  more  or  less  close  union  to  form  an  asymmetrical 
bivalent,  which  in  side  view  is  seen  to  form  a  tetrad  (Figs.  4,  /-/, 
ky  Photo  3).  This  bivalent  undergoes  an  equation  division,  in 
this  respect  agreeing  with  the  conditions  uniformly  seen  in  Syro- 


FIG.  6 

M.  femoratus  (No.  29)  22-chromosome  form 

Four  nuclei  from  growth-period  showing  diffused  ordinary  chromosomes,  condensed  chromosome- 
nucleoli  and  plasmosome;  in  a  and  b  the  two  idiochromosomes  are  united  to  form  double  chromosome- 
nucleoli  (Photo  25);  in  c  and  d  they  are  separate  (Photo  26). 

mastes  (Gross  '04,  Wilson  '09),  and  differing  from  that  occurring  in 
the  Coleoptera  or  Diptera  (Stevens  '06,  'o8a).  A  rarer  but  more 
interesting  deviation  from  the  type  is  the  failure  of  the  idiochro- 
mosomes to  separate  in  the  second  division,  both  passing  together 
to  the  same  pole  (Fig.  5,  o,  />,  Photo  18).  Since  the  other  chromo- 
somes divide  equally  it  may  be  inferred  that  in  this  case  one  pole 
receives  12  chromosomes  and  the  other  but  10.  This  has  been 
seen  in  only  three  cells  and  is  doubtless  an  abnormality.  It  may 
however,  possess  a  high  significance  as  forming  a  possible  point 


Studies  on  Chromosomes  167 

of  departure  for  the  origin  of  the  whole  series  of  relations  observed 
in  the  genus. 

3     Individuals  possessing  twenty-three   Chromosomes;  one 
Supernumerary 

This  condition  exists  in  all  three  species  and  has  been  found  in 
seven  males  and  four  females.  In  four  of  these  males  the  super- 
numerary is  large  (of  approximately  the  same  size  as  the  small 
idiochromosome,  as  in  Fig.  i,  £-/');  in  three  it  is  no  larger  than  the 
ra-chromosomes  (as  in  Fig.  I,  /-/),  and  is  indistinguishable  from 
the  latter  save  in  behavior.  In  each  case,  as  already  described, 
the  spermatogonia  show  23  chromsomes  and  the  first  division  13; 
and  in  those  showing  a  small  supernumerary  in  the  first  division 
the  spermatogonia  always  show  three  very  small  chromosomes. 

The  grouping  in  the  first  division,  though  conforming  to  the 
same  general  type,  shows  many  variations  of  detail,  as  may  be  seen 
from  Fig  7,  a-l,  Photos  4-6.  It  is  a  curious  fact  that  the  form  of 
grouping  is  to  some  extent  characteristic  of  the  individual.  For 
example,  the  typical  arrangement,  with  both  idiochromosomes 
and  supernumerary  outside  the  ring,  is  very  common  in  Nos.  43 
(Fig.  i,  /-/)  and  20  (7,  a-c),  very  rare  in  Nos.  I,  2  (Fig.  7,  /)and 
49  (Fig.  7,  f-h).  In  No.  49,  very  many  of  the  first  division  meta- 
phases  show  both  supernumerary  and  small  idiochromosome 
lying  inside  the  ring  (Fig.  7,  g-h).  I  am  unable  to  suggest  an 
explanation  of  this. 

In  this  division  all  the  chromosomes  divide  equally  (Fig.  7,  m-p), 
so  that  each  secondary  spermatocyte  receives  13  chromosomes. 
The  usual  regrouping  now  takes  place,  and  the  idiochromosomes 
couple  as  usual  to  form,  an  asymmetrical  bivalent.  The  super- 
numerary sometimes  remains  free  (i.  e.,  not  attached  to  any  other), 
in  which  case  12  chromosomes  appear  in  polar  view  (Fig.  8,  b,d). 
Much  more  frequently  the  supernumerary  attaches  itself  to  the 
idiochromosome  bivalent  to  form  a  triad  element,  polar  views  now 
showing  but  II  chromosomes  (8,  a,  c\  one  of  which  is  compound. 
The  three  components  of  such  triads  usually  lie  in  a  straight  line, 
the  supernumerary  being  attached  sometimes  to  the  small  idio- 


1 68  Edmund  B.    Wilson 


FIG.  7 
23-chromosome  forms,  one  supernumerary 

a-h,  first  division,  polar  views,  one  large  supernumerary;  a-c,  term.,  No.  20,  typical  grouping;  d—e< 
gran.,  No.  48;  /,  g,  h,  gran.,  No.  49  (Photo  5). 

»'-/,  first  division,  polar  views,  one  small  supernumerary;  ;',  term.  No.  i  (Photo  6);  j-l,  term.  No.  43 
typical  grouping  in  k. 

m-p,  first  division,  side-views;  m  and  n  (term.  No.  43)  show  division  of  7,  /',  m,  and  small  s;  o,  term., 
No.  20,  division  of  I,  i,  and  large  s;  p,  term.,  No.  43,  division  of  m,  i,  and  small  s. 

cf-s,  spermatogonial  groups  from  individuals  with  one  large  supernumerary;  q,  r,  term.,  No.  20;  s, 
gran.,  No.  49. 

t-y,  spermatogonial  groups  from  individuals  with  one  small  supernumerary;  /,  u,  term.,  No.  43; 
v-y,  term.,  No.  2  (Photo  29). 


Studies  on  Chromosomes 


169 


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170 


Edmund  B.    Wilson 


chromosome,   sometimes  to  the  large,  or  not  infrequently  lying 
between  the  two  (Fig.  8,  g,  h,  o-q}. 


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FIG.  8 

2 3- chromosome  forms,  one  supernumerary 

a-f,  polar  views,  second  division;  a,  gran.,  No.  49,  large  supernumerary  attached;  fe(same  cyst)super- 
numerary  free;  c-d,  similar  views  of  terminalis,  No.  43,  with  small  supernumerary;  e-f  (No.  43),  sister 
groups  from  same  spindle,  pclar  views. 

g-m,  side-views,  second  division,  from  gran.,  No.  49,  with  large  supernumerary,  free  in  j,  attached 
in  the  others. 

n-u,  similar  views  from  individual  (term.,  No.  43)  with  small  supernumerary;  in  u  the  supernumer- 
ary is  free. 


Studies  on  Chromosomes  171 

In  the  ensuing  division,  if  the  supernumerary  lies  free  it  passes 
without  division  as  a  heterotropic  chromosome  to  one  pole  (8,  «). 
When  connected  with  the  idiochromosome  bivalent  it  passes  to  one 
pole  attached  to  one  or  the  other  of  the  idiochromosomes  (Fig. 
8,  k-m,  p-t).  In  either  case  one  pole  receives  1 1  chromosomes  and 
one  12  (Fig.  8,  e,  /);  but  since  the  supernumerary  may  accompany 
either  idiochromosome  four  classes  of  spermatid  nuclei  are  formed, 
namely: 

(l)       10  =7=11  (2)       10  +  /  +  S  =    12 

(3)      10  +  t  --=    II  (4)      10  +  I  +  S  =    12 

As  described  in  an  earlier  paper  ('oya),  there  is  a  tendency  for 
the  supernumerary  to  be  associated  more  often  with  the  small 
idiochromosome  than  with  the  large,  and  classes  I  and  2  are  accord- 
ingly more  numerous  than  3  and  4.  I  was  formerly  inclined  to 
attribute  importance  to  this  as  pointing  to  the  more  frequent 
occurrence  of  the  supernumerary  in  the  male  than  in  the  female. 
The  larger  series  of  data  now  available  leads  me  to  doubt  whether 
it  has  much  significance;  for  if  (leaving  the  2i-chromosome  forms 
out  of  account)  the  whole  series  of  forms  be  taken  together,  one 
or  more  supernumeraries  are  found  in  27  out  of  34  males,  and  in 
15  out  of  19  females — about  80  per  cent  in  each  case.  It  appears 
therefore  that  in  the  long  run  the  supernumeraries  are  distributed 
between  the  two  sexes  with  approximate  equality. 

Figs.  7,  q-s  show  spermatogonial  groups  from  individuals  with 
one  large  supernumerary,  but  in  none  of  them  can  this  chromosome 
or  the  small  idiochromosome  be  certainly  distinguished.  Fig. 
7,  t-y  are  from  individuals  with  one  small  supernumerary,  each 
showing  three  very  small  chromosomes.  In  t  and  u  the  small 
idiochromosome  is  doubtful.  Fig.  7,  v-y,  on  the  other  hand,  are 
from  an  individual  (terminalis,  No.  2),  showing  great  numbers  of 
very  fine  spermatogonial  groups,  in  almost  all  of  which  the  small 
idiochromosome  is  at  once  recognizable.  The  same  is  true  of  a 
second  individual  from  the  same  locality.  These  two  individuals, 
from  the  Paulmier  collection,  were  the  first  material  I  examined 
and  found  so  puzzling  until  the  examination  of  another  similar 
individual,  No.  43,  cleared  up  the  nature  of  the  second  division. 


172  Edmund  B.    Wilson 

4     Individuals  with  twenty-six  Chromosomes;  four 
Supernumeraries 

It  will  be  convenient  to  consider  this  type  before  the  24-  and  25- 
chromosome  forms,  since  the  material  is  more  favorable  for  an 
account  of  the  remarkable  phenomena  occurring  in  the  second 
division.  Of  these  individuals  there  are  seven  males  and  three 
females,  all  three  species  being  represented.  Unfortunately  very 
few  perfectly  clear  spermatogonial  groups  are  shown;  but  the 
spermatocyte-divisions  and  cells  of  the  growth-period  are  particu- 
larly well  shown  and  in  large  numbers  of  cells.  In  all  but  one  of 
these  individuals  the  four  supernumeraries  are  large  and  of  nearly 
equal  size.  In  one  (femoratus  No.  40)  two  are  large  and  two 
small.  The  latter  case,  already  shown  in  Fig.  2,  /-/,  is  further 
illustrated  by  Fig.  9,  A,  /',  /,  n,  o.  Two  of  these  (h  and  /)  show 
but  three  supernumeraries  in  the  first  division,  a  common  appear- 
ance in  this  individual  (see  p.  186).  Fig.  9,  a-/,  show  varying 
arrangements  of  the  16  chromosomes  that  appear  in  the  first 
division,  the  most  typical  ones  being  k  and  /.  In  9,  a-c,  k,  /,  both 
idiochromosomes  and  the  four  supernumeraries  lie  outside  the 
ring.  In  9,  g,  all  but  the  large  idiochromosome  are  inside  the 
ring. 

In  some  of  these  slides  the  compound  chromosome-nucleoli  are 
shown  with  great  distinctness  in  many  cells  of  the  growth-period. 
This  body  usually  has  the  form  of  a  flat  plate  that  lies  next  the 
nuclear  wall  (Fig.  10,  q,  r)  so  that  a  clear  view  of  all  the  compo- 
nents can  only  be  had  in  tangential  sections.  Thus  viewed  (Fig. 
10,  s-u,  Photo  28)  it  may  often  be  seen  to  consist  of  six  components 
one  of  which  (the  large  idiochromosome)  is  about  twice  the  size  of 
the  others  and  is  usually  at  one  side  or  end  of  the  group.  The 
other  five  evidently  represent  the  small  idiochromosome  and 
the  four  supernumeraries.  In  side  view  (Fig.  10,  q,  r)  not  more 
than  three  or  four  of  the  components,  can  as  a  rule  be  recognized. 
In  a  considerable  number  of  cases  these  six  chromosomes  are  not 
aggregated  to  form  a  single  body  but  form  two  or  more  simpler 
bodies. 

The  second  division  in  these  forms  presents  an  extraordinary 


Studies  on  Chromosomes 


173 


•**••   ,» 


»**«&   ft  • 


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h 


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r{ 


m 


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J 


'/ 


FIG.  9 

26-chromosome  forms,  four  supernumeraries 
a-g,  first  polar,  supernumeraries  large  and  equal;  a-d,  fern.,  No.  42;  e,  gran.,  No.  55; /,  gran.,  No. 

59!  &  gran.,  No.  60. 

h-j  first  polar,  from  (fern.,  No.  40,  with  two  large  supernumeraries  and  two  small;  all  of  these  are 
shown  in;',  (cf.  Fig.  2,;),  while  in  h  and  /  one  is  missing  (see  p.  186). 

k,  first  polar,  term.,  No.  36;  /  from  same  individual  (Photo  9). 

m-o,  spermatogonia  groups;  m,  fern.,  No.  42,  abnormal  group  with  27  chromosomes;  n,  o,  fern., 
No.  40.  showing  two  small  supernumeraries. 

p-q,  ovarian  groups,  gran.  No.  61. 


174  Edmund  B.    Wilson 

appearance  which  I  at  first  thought  must  be  due  to  an  artificial 
clumping  together  of  the  chromosomes  through  defective  fixation; 
but  the  study  of  very  many  of  these  figures  convinced  me  that  such 
is  not  the  case.  As  in  the  preceding  types,  ten  of  the  chromosomes, 
including  the  m-chromosomes,  have  the  form  of  symmetrical 
dumb-bell  shaped  bodies  which  are  equally  halved  in  the  ensuing 
division.  The  remaining  chromosomes  are  usually  aggregated  to 
form  a  compound  element  (Fig.  10,  /z-/,  Photos  22,  23)  in  which 
may  be  very  clearly  distinguished  the  same  components  as  those 
that  appear  in  the  chromosome-nucleoli  of  the  growth-period; 
and  the  size-relations  make  it  evident  that  one  of  them  is  the  large 
idiochromosome,  one  the  small,  while  four  are  the  supernumer- 
aries. In  other  words,  these  six  chromosomes,  which  divide  as 
separate  univalents  in  the  first  division,  have  now  again  conju- 
gated to  form  a  hexad  group.  This  compound  element  almost 
always  lies  near  the  center  of  the  group.  Polar  views  of  this  divi- 
sion accordingly  show  typically  n  chromosomes,  of  which  the 
central  one  is  compound  (Figs.  10,  a-g,  Photo  13).  Not  infre- 
quently, however,  one  or  more  of  the  supernumeraries  may  be  sep- 
arate from  the  others  (Fig.  10,  f,  g),  the  apparent  number  in  polar 
view  varying  accordingly. 

In  side  views  the  grouping  of  the  components  of  the  hexad 
element  is  seen  to  vary  considerably  though  the  large  idiochro- 
mosome is  more  frequently  at  one  end  of  the  group.  In  the  ensu- 
ing division  the  other  ten  chromosomes  divide  equally,  while  the 
hexad  element  breaks  apart  into  two  groups  that  pass  to  opposite 
poles  (Fig.  10,  /-/>).  The  distribution  of  the  various  elements 
is  difficult  to  determine  exactly,  since  they  always  lag  behind  the 
others  in  the  anaphases  and  are  scattered  along  the  spindle  in  such 
a  way  as  often  to  give  confusing  pictures.  The  study  of  many  such 
anaphases  leads  me  to  conclude,  however,  that  at  least  one  of  the 
smaller  components  always  passes  to  the  opposite  pole  from  the 
larger  one,  while  the  other  four  undergo  a  variable  distribution. 
In  Fig.  10,  /,  the  group  is  just  separating  into  three  toward  each 
pole;  in  10  m,  it  is  quite  clear  that  three  of  the  small  ones  are  pass- 
ing to  one  pole,  while  the  large  one  and  two  small  ones  are  passing 
to  the  other,  and  Fig.  10,  n,  is  probably  a  similar  case.  In  these 


Studies  on  Chromosomes  175 

cases  it  seems  clear  that  each  pole  receives  13  chromosomes,  as 
follows : 

a     10  +  7  +  2*  =  13  b     10  +  i  +  2s  =  13 

Fig.  10,  o,  on  the  other  hand,  shows  a  perfectly  clear  case  in 
which  the  hexad  element  has  separated  into  a  2-group  and  4-group: 
Fig.  10,  /?,  shows  what  is  probably  a  later  stage  of  the  same  type. 
In  both  these  cases  one  pole  appears  to  receive  12  and  one  14  as 
follows : 

a     10  +  /  +  3*  +  14  b     10  +  ;  +  s  =  iz 

one  pole  receiving  but  one  supernumerary,  and  the  other  three. 
The  cases  in  which  all  of  the  components  may  be  clearly  recog- 
nized in  the  anaphases  are  comparatively  rare,  and  in  the  greater 
number  of  them  the  distribution  of  the  supernumeraries  appears  to 
be  symmetrical.  Of  their  unsymmetrical  distribution  in  some 
cases  there  can  be  no  doubt  (and  the  same  is  true  of  the  T4-chromo, 
some  form,  as  described  beyond).  The  few  undoubted  cases  of 
this  all  show  one  to  one  pole  and  three  to  the  other  (as  in  Fig.  10,  o- 
p),  and  I  have  never  found  a  case  in  which  all  four  pass  to  the  same 
pole. 

It  seems,  therefore,  probable  that  in  the  26-chromosome  type 
there  are  at  least  six  classes  of  spermatozoa,  as  follows: 

(l)  10  +  I  +  2s  =  13  (l)  10  +  ;  +  2s  —  13 

(3)   10  +  7  +   S  =  12  (4)   10  +  I  +  35  =  14 

(5)     10  +  7  +  3*  =  14  (6)    10  +  /  +    j  =  12 

It  is  possible  that  the  following  four  additional  classes  may  be 
produced : 

(7)     10  +  7  +  4*  =  15  (8)     10  +  ;  =n 

(9)    10  +  /          =11  (10)    10  +  »  +  45  =  15 

Perfectly  clear  spermatogonial  figures  of  this  type  were  rarely 
found,  though  many  of  them  show  approximately  26.  The  nor- 
mal group  of  fern.,  No.  42,  is  shown  in  Fig.  2,  h.  Two  groups  from 
fern.  No.  40  (with  two  small  and  two  large  supernumeraries)  are 
shown  in  Fig.  9,  n,  o,  each  having  26  chromosomes  including  four 
small  ones  (cf.  Fig.  2,  k).  Two  ovarian  groups  from  gran.,  No.  61, 


176  Edmund  B.    Wilson 


FIG.  10 

26-chromosome  forms 

tt-gj  second  division,  polar,  d  from  fern.  No.  40,  the  others  from  fern.  No.  42;  a,  (Photo  13)  b,  c, 
show  a  single  central  hexad;  in  e  and  g  the  components  are  more  loosely  united;  in  d  and  /one  supernumer- 
ary is  free. 

h-p,  side-views,  second  division,  from  fern.  No.  42  (Photos  22,  23)  explanation  in  text. 
q-u,  growth-period,  gran..  No.  60;  q  and  r  show  the  compound  chromosome-nucleolus  in  oblique 
and  side-view,  s,  t,  u,  en  face. 


Studies  on  Chromosomes 


'77 


178  Edmund  B.    Wilson 


FIG.  n 

24-chromosome  forms,  two  supernumeraries. 

a-e,  term.,  No.  21,  first  polar,  showing  various  groupings;  g,  the  same,  gran.,  No.  52  (Photo  7). 

h,  term.,  No.  21,  second  polar,  tetrad  element  near  center. 

i-o,  somatic  groups  from  individuals  with  two  large  supernumeraries;  /-/,  spermatogonial  groups 
from  term.  No.  21;  m,  n,  ovarian  groups  from  fern.  No.  3150,  ovarian  group,  fern.,  No.  45. 

p-r,  spermatogonial  groups  from  fern.,  No.  22,  with  one  large  supernumerary  and  one  small;  Photo 
30). 

s-w,  second  division,  side-view;  st  term.,  No.  21 ;  t-w,  gran.,  No.  52  (Photo  21). 


Studies  on  Chromosomes 


179 


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180  Edmund  B.   Wilson 

are  shown  in  Fig.  9,  /?,  q.  Fig.  9,  m,  shows  a  spermatogonial  group 
from  fern.,  No.  42,  that  is  abnormal  in  showing  with  perfect  clear- 
ness 27  instead  of  26  chromosomes  (cf.  Fig.  2,  /z). 

5     Individuals  with   twenty-four   Chromosomes;  two 
Supernumeraries 

The  material  for  these  individuals  and  those  of  the  25-chromo- 
some  class,  is  less  satisfactory  than  in  the  preceding  case,  but  the 
relations  are  undoubtedly  quite  analogous  to  those  just  described. 
The  24-chromosome  class  is  represented  by  9  males  and  4  females, 
and  occurs  in  all  three  species.  In  one  of  the  males  one  of  the 
supernumeraries  is  large  (of  the  same  size  as  a  small  idiochromo- 
some)  and  one  small;  in  all  the  others  both  are  large.  Additional 
figures  of  the  first  division,  showing  variations  in  the  grouping,  are 
given  in  Fig.  II,  a-g;  of  spermatogonial  groups  in  Figs,  n,  i-r. 
Of  particular  interest  is  the  male,  term.,  No.  22,  shown  in  Photo  30 
and  in  Fig.  1 1,  p-r.  This  individual  was,  unfortunately,  immature 
showing  only  spermatogonia  and  cells  in  the  growth-period;  but 
many  perfectly  clear  spermatogonial  groups  are  shown.  These 
groups  uniformly  show  24  chromosomes,  of  which  three  are  very 
small,  while  in  many  cases  two  others  are  slightly  but  distinctly 
smaller  than  the  others.  The  latter  are  evidently  the  small  idio- 
chromosome  and  the  larger  supernumerary,  while  the  three  small 
ones  represent  the  w-chromosomes  and  the  small  supernumerary. 

In  the  second  division  the  two  idiochromosomes  and  the  super- 
numeraries are  frequently  united  to  form  a  tetrad  element,  various 
forms  of  which  are  shown  in  Fig.  n,  s-w.  The  distribution  of 
these  four  components  is  not  so  well  shown  in  this  material  as  in 
that  of  the  26-chromosome  class,  described  above.  It  is,  however, 
clear  that  this  distribution  is  inconstant.  In  cases  like  those  shown 
in  Fig.  n,  j,  t,  it  is  probable  that  the  tetrad  divides  in  the  middle, 
so  that  each  idiochromosome  is  accompanied  by  a  supernumerary, 
and  each  pole  receives  12  chromosomes.  The  cases  shown  in 
Fig.  n,  <y,  w,  prove  however  that  this  is  not  always  the  case;  for 
in  w  the  large  idiochromosome  is  seen  passing  to  one  pole  while 
both  supernumeraries,  attached  to  the  small  idiochromosome, 


Studies  on  Chromosomes  181 

are  passing  to  the  other.  In  this  case  one  pole  receives  1 1  chromo- 
somes, the  other  13.  It  is  evident  that  in  this  form  there  is  the 
possibility  of  forming  six  classes  of  spermatozoa,  as  follows: 

(l)       10  +  7  =    II  (2)       IO  +   I   +   25  =    13 

(3)       10  +  7  +      S  =    12  (4)       10  +  »  +      S=    12 

(5)     10  +  /  +  2s  =  13  (6)     10  +  /     =  ii 

In  none  of  these  individuals  is  the  material  very  favorable  for 
the  study  of  the  chromosome-nucleoli.  They  are  always  evidently 
compound,  but  only  in  a  few  cases  can  the  components  be  clearly 
recognized  (as  in  Fig.  2,  c). 

6  Individuals  with    twenty-five  Chromosomes;  three 

Supernumeraries 

No  individuals  of  this  type  were  found  in  M.  femoratus.  The 
other  two  species  are  represented  by  three  males  and  three  females 
but  here  again  the  material  does  not  admit  of  exhaustive  study. 
In  one  of  the  females,  two  of  the  supernumeraries  are  large  and 
one  small,  the  ovarian  cells  showing  25  chromosomes,  of  which 
three  are  very  small  (Fig.  12,  i-k),  a  condition  seen  in  every  group 
of  which  a  clear  view  can  be  had.  The  two  larger  supernumer- 
aries cannot,  however,  be  certainly  identified  in  any  of  these.  In 
all  the  other  individuals  the  supernumeraries  are  of  the  larger 
form.  Fig.  12,  a,  b,  show  the  first  division  in  one  of  these  cases 
(term.,  No.  34);  c-g  are  spermatogonial  groups  from  the  same  indi- 
vidual; h,  an  ovarian  group  of  the  same  type.  Fig.  12,  m-p,  are 
from  a  doubtful  case  in  which  nearly  all  the  first  division  figures 
show  three  supernumeraries  (n,  o),  but  a  single  case  (m)  shows 
distinctly  four. 

7  Individuals  with  twenty-seven   Chromosomes;  five 

Supernumeraries. 

This  class  is  represented  by  a  single  very  interesting  male  of 
granulosus  (No.  57),  in  which  only  the  first  division  can  be  satisfac- 
torily examined.  Many  polar  views  of  this  division  show  17 
chromosomes  (Fig.  13,  a-/,  Photo  10),  of  which  two  are  always 


i82 


Edmund  B.    Wilson 


smaller  than  the  others.  One  of  these,  always  central  in  position, 
is  evidently  the  m-chromosome  bivalent.  Of  the  remaining  six, 
one  is  in  most  cases  decidedly  smaller  than  the  others — a  relation 


a 


f 


m 


n 


FIG.   12 

25-chromosome  forms,  three  supernumeraries 

a,  b,  first  polar,  term.,  No.  34;  c-g,  spermatogonial  groups  from  same  individual;  h,  term.,  No.  38. 
ovarian  group;  i-k,  ovarian  groups  from  term.,  No.  27,  with  two  large  supernumeraries  and  one  small; 
/,  gran.,  No.  58,  ovarian  groups,  three  large  supernumeraries. 

m,  n,  o,  first  division,  p,  second  division  from  gran.,  No.  54,  with  three  or  four  supernumeraries. 

of  which  the  constancy  is  attested  by  the  nine  figures  given  of  this 
division.  It  is  evident  that  in  this  individual  there  are  four  large 
supernumeraries  and  one  small;  and  although  nospermatogonia 


Studies  on   Chromosomes  183 

are  clearly  shown  it  may  be  inferred  that  the  somatic  number  is  27. 
The  chromosome-nucleoli  in  this  individual  are  evidently  com- 
pound, but  in  no  case  can  all  the  components  be  clearly  recognized. 
The  second  division  shows,  as  a  rule,  n  elements  in  polar  view, 
the  central  one  being  compound  (Fig.  13,  j-k),  but  the  distribu- 
tion of  the  compound  element  could  not  be  determined. 

•l*li  v*:  **v 

*,   ^  §•*,  *•  ^  5  •* 


abed 

ff      *+.*? 

e        ***f  g        *~h 


•T-  •  %>   , 

/  •  J  W    k 

FIG.  13 

27  and  ( ?)  zS-chromosome  forms 

a-i,  first  division,  from  gran.,  No.  57,  having  four  large  supernumeraries  and  one  small  j  (polar) 
and  k  (side-view),  second  division,  same  individual  (Photo  10). 

/,  ovarian  group  from  fern.,  No.  33,  having  three  large  and  two  or  three  small  supernumeraries;  in 
this  group  appear  28  chromosomes. 

8     Individuals  with   twenty-eight  (?)   Chromosomes;  six 
Supernumeraries. 

The  last  case  to  be  considered  is  that  of  a  single  female  of  femora- 
tus  (No.  33),  in  which  the  number  is  either  27  or  28.  A  single 
perfectly  clear  ovarian  group,  shown  in  Fig.  13,  /,  shows  beyond 


184  Edmund  B.    Wilson 

doubt  28  chromosomes,  including  five  smallest  ones  and  three  or 
four  next  smallest.  A  few  other  less  clear  groups  were  seen  in 
which  appear  but  27  chromosomes,  the  missing  one  being  one  of 
the  smallest.  In  these  cases  one  of  the  small  ones  may  be  hidden 
among  the  larger  ones;  but  it  is  also  possible  that  the  28-group  is  an 
abnormality.  In  this  individual  there  are  probably  three  larger 
supernumeraries  and  either  two  or  three  small  ones. 

C      SUMMARY   AND    CRITIQUE 

1  In  the  genus  Metapodius  the  number  of  chromosomes  is 
constant  in  the  individual  but  varies  in  different  individuals  from 
21  to  27  or  28.     The  number  21  appears  only  in  the  males  of  M. 
terminalis  (Montgomery's  material). 

2  The  number  is  independent  of  sex  and  locality,  and  is  not 
correlated  with  constant  differences  of  size  or  visible  structure  in 
the  adults. 

3  The  variation  affects  only  a  particular  class  of  chromosomes. 

4  The  22-chromosome  forms  represent  the  type  from  which  all 
the  others  may  readily  be  derived.     These  forms  possess  a  pair  of 
unequal  idiochromosomes  which  show  the  same  behavior  as  in 
Lygaeus  or  Euschistus,  all  the  spermatozoa  receiving  n  chromo- 
somes, and  half  containingthe  large  idiochromosome,  half  the  small. 

5  In  the  2i-chromosome  forms  the  small  idiochromosome  has 
disappeared,  leaving  the  large  one  as  an  "odd"  or  "accessory" 
chromosome.     Half  the  spermatozoa  accordingly  receive  1 1  chro- 
mosomes and  half  10. 

6  Numbers  above  22  are  due  to  the  presence  of  from  one  to  five 
or  six  additional  small  chromosomes  which  show  in  every  respect 
the  same  behavior  as  the  idiochromosomes,  and  are  probably  to  be 
regarded  as  additional  small  idiochromosomes.     In  the  growth 
period  they  have  a  condensed  form  and  are  typically  associated 
with   the   idiochromosomes   to  form   a  compound   chromosome- 
nucleolus.     In  the  first  division  they  divide  as  separate  univalents. 
In  the  second,  they  are  typically  (though  not  invariably)  again 
associated  with  the  idiochromosomes  to  form  a  compound  element. 
The  components  of  this  element  undergo  a  variable  distribution 


Studies  on  Chromosomes  185 

to  the  spermatid  nuclei.  All  the  spermatid  nuclei  receive  the 
haploid  type-group  of  II  chromosomes,  half  including  the  small 
idiochromosomes  and  half  the  large;  but  in  addition  each  may 
receive  one  or  more  supernumeraries.  The  total  number  of 
chromosomes  in  the  sperm  nuclei  is  therefore  variable  in  the  same 
individual. 

7  Both  the  number  of  the  supernumeraries  and  their  size,  indi- 
vidually considered,  are  constant  in  the  individual. 

The  first  question  that  the  foregoing  report  of  results  will  raise 
is  whether  the  number  and  size  relations  of  the  chromosomes  in 
each  individual  are  really  as  constant  as  I  have  described  them. 
I  have  for  the  most  part  selected  for  illustration  and  description 
the  more  typical  conditions ;  but,  granting  the  accuracy  of  the  figures, 
does  such  a  selection  really  give  a  fair  presentation  of  the  actual 
conditions  ?  It  is  almost  needless  to  say  that  very  many  cases 
might  have  been  shown  that  would  seem  to  give  conflicting  results. 
By  far  the  greater  number  of  these  discrepancies  are,  I  believe, 
only  apparent.  Numerical  discrepancies  of  this  kind  are  very 
often  evidently  due  to  mere  accidents  of  sectioning  or  to  the  super- 
position or  close  contact  of  two  or  more  chromosomes.  Again, 
apparent  discrepancies  in  the  size  relations  of  the  chromosomes,  as 
seen  in  polar  views,  very  often  arise  through  different  degrees  of 
elongation  (particularly  in  the^  maturation  divisions).  But  apart 
from  such  apparent  variations,  real  deviations  undoubtedly  occur 
in  almost  all  of  the  relations  described.  Now  and  then,  for  exam- 
ple, a  spermatogonial  or  ovarian  group  is  found  that  clearly  shows 
one  chromosome  too  many  (as  in  Fig.  9,  m),9  and  the  same  is  true 
of  the  first  spermatocyte-division,  but  such  cases  are  very  rare. 
The  former  case  is  probably  a  result  of  an  abnormality  in  the  forma- 
tion of  the  chromosomes  from  the  resting  nucleus,  the  latter  not 
improbably  to  a  failure  of  synapsis.  Again,  both  spermatogonial 
and  spermatocyte-cysts  are  occasionally  found  in  which  the  num- 
ber of  chromosomes  is  doubled  or  quite  irregular.  These  are 

9  A  perfectly  clear  case  of  this  has  been  found  in  the  pyrrochorid  species  Largus  cinctus  (a  particu- 
larly fine  form  for  study).  In  this  form  the  normal  male  number  is  n,  the  female  12;  but  in  one 
female  three  cells  were  found  each  of  which  shows  with  all  possible  clearness  13  chromosomes,  very 
many  other  cells  showing  the  normal  number. 


1 86  Edmund  B.    Wilson 

probably  due  to  an  antecedent  nuclear  division  without  cell  divi- 
sion, or  to  multipolar  mitoses  such  as  now  and  then  occur  in  both 
spermatogonia  and  spermatocytes. 

As  regards  the  chromosome-nucleoli  of  the  growth-period,  the 
contrast  between  those  of  the  21  and  22-chromosome  forms,  or 
between  either  of  these  forms  and  those  with  higher  numbers  is 
usually  at  once  apparent;  but  in  very  many  cases  where  more  than 
one  supernumerary  is  present  the  number  of  components  can  only 
here  and  there  be  clearly  seen.  Contrary  to  what  might  be  expected 
from  their  compact  form,  the  compound  chromosome  nucleoli 
seem  to  be  rather  difficult  of  proper  fixation,  their  components 
often  clumping  together  or  breaking  up  more  or  less  when  they 
coagulate.  I  infer  this  from  the  fact  that  different  slides  differ 
materially  in  the  clearness  with  which  these  bodies  are  shown. 

Two  discrepancies,  apparent  or  real,  should  be  especially  men- 
tioned. One  is  the  difficulty  of  recognizing  the  larger  supernu- 
meraries in  the  somatic  groups.  As  already  explained,  these  chro- 
mosomes, like  the  idiochromosomes,  appear  relatively  much 
larger  in  the  somatic  groups  than  in  the  first  maturation  division 
(owing  to  their  univalence  in  the  latter  case) ;  but  we  should  expect 
to  recognize  them  more  clearly,  at  least  in  the  female  groups,  than 
is  actually  the  case.  This  is  perhaps  due  to  their  undergoing  a 
greater  degree  of  condensation  than  the  others  during  the  growth- 
period;  but  I  am  not  sure  that  this  explanation  will  suffice.  A 
second  discrepancy,  which  may  involve  an  important  conclusion, 
is  that  in  perfectly  clear  views  of  the  first  division,  the  number  of 
supernumeraries  is  often  less  than  would  be  expected  from  the 
spermatogonial  groups.  This  is  notably  the  case  with  femoratus, 
No.  40  (Fig.  9,  h-f),  which  has  clearly  26  spermatogonial  chro- 
mosomes, but  very  rarely  shows  16  in  the  first  division,  the  usual 
number  being  15.  A  similar  discrepancy  has  been  noted  in  other 
individuals,  and  in  several  of  the  types.  Since  the  typical  num- 
ber in  all  these  cases  appears  in  some  or  many  of  the  first  sperma- 
tocytes, I  long  supposed  the  occasional  deficiency  to  result  from  an 
accident  of  sectioning.  I  now  incline  to  believe,  however,  that  in 
some  cases  one  (or  possibly  more)  of  the  supernumeraries  may 
really  disappear  (by  degeneration  ?)  during  the  growth-period, 


Studies  on  Chromosomes  187 

and  that  this  may  be  one  way  in  which  their  progressive  accumu- 
lation in  number  in  successive  generations  is  held  in  check. 

For  the  foregoing  reasons  it  cannot  be  said  that  any  of  the  rela- 
tions described  appear  with  absolute  uniformity  or  fixity.  The 
condition  typical  of  each  individual  must  be  discovered  by  the 
study  and  comparison  of  large  numbers  of  cells.  I  will  only  say 
that  prolonged  and  repeated  study  has  thoroughly  convinced  me 
that  the  relations,  as  described,  may  be  regarded  as  being  on  the 
whole  individual  constants.  This  judgment  is  based  primarily 
on  the  exhaustive  study  of  a  few  of  the  best  series  of  preparations 
of  individuals  of  the  21,  22,  23,  and  26-chromosome  types,  in 
which  the  facts  are  quite  unmistakable  and  have  given  the  point 
of  view  from  which  the  less  favorable  material  of  other  cases  may 
fairly  be  examined. 

D      DISCUSSION    OF    RESULTS 

The  principal  significance  of  these  phenomena  seems  to  me 
to  lie  in  their  bearing  on  the  general  hypothesis  of  the  "individual- 
ity" or  genetic  continuity  of  the  chromosomes;  but  they  are  also 
of  interest  for  a  number  of  more  special  problems  which  I  will 
first  briefly  consider. 

The  Relation  of  the  Chromosomes  to  Sex-production  in  Metapodius 

The  conditions  seen  in  this  genus  seem  to  be  irreconcilable  with 
any  view  that  ascribes  the  sexual  differentiation  to  a  general  quanti- 
tative difference  of  chromatin,  whether  expressed  in  the  number  or 
the  relative  size  of  the  chromosomes.  In  all  known  cases  of  constant 
sexual  differences  in  the  chromosomes  it  is  invariably  the  female 
that  possesses  the  larger  number  of  chromosomes  or  the  greater 
quantity  of  chromatin,10  and  this  has  naturally  suggested  the 
view  that  this  difference  per  se  may  be  the  sex-determining  factor. 
As  I  have  pointed  out  before  ('09),  such  a  view  is  inapplicable  to 
cases  like  Nezara  or  Oncopeltus,  where  the  idiochromosomes  are 
of  equal  size  and  no  quantitative  sexual  differences  are  visible; 
yet  the  phenomena  in  these  genera  are  otherwise  so  closely  similar 

10  See  review  in  Wilson  '09. 


1 88  Edmund  B.    Wilson 

to  those  seen  in  other  insects  that  I  cannot  doubt  their  essential 
similarity  also  in  respect  to  sex-production. 

In  Metapodius  the  facts  are  still  more  evidently  opposed  to  the 
quantitative  interpretation.  The  number  of  chromosomes  has 
here  no  relation  to  sex-production;  and,  as  will  be  seen  from  the 
table  at  p.  149,  in  the  forms  with  supernumeraries  the  relative 
frequency  of  high  numbers  and  of  low  is  nearly  equal  in  the  two 
sexes.  If  my  general  interpretation  of  the  chromosomes  in  this 
genus  be  correct,  a  like  conclusion  applies  to  the  total  relative 
mass  of  chromatin  in  the  two  sexes;  for  all  individuals  alike  possess 
the  type-group  of  22  chromosomes  (Montgomery's  form  excepted) 
while  the  supernumeraries  represent  the  excess  above  this  amount. 
I  have  endeavored  to  determine  whether  this  appears  in  direct 
measurements,  independently  of  my  general  interpretation;  but 
have  found  this  impracticable  for  several  reasons.  Very  consider- 
able differences  in  the  apparent  size  of  the  chromosomes  are  pro- 
duced by  different  degrees  of  extraction;  but  this  will  not  account 
for  the  considerable  differences  seen  in  the  same  slide  when  the 
extraction  is  uniform.  It  is  evident  that  the  actual  size  of  the 
chromosomes  varies  with  the  size  of  the  cells;  for  example,  both 
in  Metapodius  and  in  many  other  genera,  the  chromosomes  in 
the  larger  spermatogonia  near  the  tip  of  the  testis  are  larger 
(in  many  cases  much  larger)  than  those  of  the  smaller  spermato- 
gonia of  other  regions.  How  great  the  differences  are  may  be 
appreciated  by  a  comparison  of  the  figures.  For  example,  in 
the  spermatogonial  groups  of  No.  2  (23  chromosomes,  Fig.  7, 
•u-x),  the  chromatin  mass  is  obviously  much  greater  than  in 
those  of  No.  21  (24  chromosomes,  Fig.  II,  /-/).  In  the  25-chromo- 
some  female  groups  shown  in  Fig.  12,  i-k  (No.  27),  the  chromatin 
mass  is  evidently  much  less  than  in  the  2i-chromosome  male  group 
shown  in  Fig.  I,  &,  or  in  the  23-chromosome  male  groups  of  Fig.  7, 
v-x.  Conversely,  the  22-chromosome  female  group  of  No.  44 
(Fig.  4,  j)  shows  a  much  greater  chromatin  mass  than  in  the  corre- 
sponding male  group  of  No.  46  (Fig.  4,  o),  or  the  male  24-chromo- 
some  group  shown  in  Fig.  n,  j. 

Evidently,  therefore,  the  relative  mass  of  chromatin  can  only 
be  determined  by  means  of  accurate  measurements  of  both  the 


Studies  on  Chromosomes  189 

chromosomes  and  the  mass  of  protoplasm,  but  I  have  found  the 
errors  of  measurement  of  the  cell  size  to  be  too  great  to  give  any 
trustworthy  result  regarding  the  relative  chromatin  mass. 

Despite  the  difficulties  in  the  way  of  an  accurate  direct  deter- 
mination, I  believe  the  facts  on  the  whole  warrant  the  conclusion 
that  the  relative  chromatin  mass  shows  no  constant  correlation 
with  sex.  The  most  probable  conclusion  is  that  the  male-produc- 
ing spermatozoa  in  Metapodius  are  distinguished  by  the  same 
characters  as  in  other  forms  having  unequal  idiochromosomes, 
the  former  class  being  those  that  receive  the  large  idiochromosome, 
the  latter  those  that  receive  the  small  one,  irrespective  of  the  super- 
numeraries that  may  be  present  in  either  class.  For  reasons  that 
I  have  elsewhere  stated,  I  believe  that  if  the  idiochromosomes  be 
the  sex-determinants  their  difference  is  probably  a  qualitative 
one,  and  since  the  small  idiochromosome  may  be  lacking  it  would 
seem  that  the  large  one  must  in  every  case  play  the  active  role — 
perhaps  as  the  bearer  of  a  specific  substance  (enzyme  ?)  that  calls 
forth  a  definite  reaction  on  the  part  of  the  developing  individual. 
If  this  be  so,  we  can  comprehend  the  fact  that  the  presence  of 
additional  small  idiochromosomes  (supernumeraries)  in  either 
sex  does  not  affect  the  development  of  the  sexual  characters  in  that 
sex. 

b     The  possible  Origin  of  the   unpaired  Idiochromosome   ("odd" 
or  "accessory"  Chromosome)  and  of  the  Supernumeraries 

The  explanation  of  the  unpaired  idiochromosome  offered  in  the 
second  and  third  of  my  "  Studies  on  Chromosomes"  ('05,  '06)  was 
suggested  by  the  fact  that  various  degrees  of  inequality  exist  in  the 
paired  idiochromosomes,  there  being  an  almost  continuous  series 
of  forms  connecting  those  in  which  the  idiochromosomes  are 
equal  (Nezara,  Oncopeltus)  with  those  in  which  they  are  so  very 
unequal  that  the  small  one  appears  almost  vestigial  (Lygaeus, 
Tenebrio).  It  is  evident  that  by  the  further  reduction  and  final 
disappearance  of  the  small  member  of  this  pair  the  large  one  would 
be  left  without  a  mate,  and  its  history  in  the  maturation  process 
would  become  identical  with  that  of  an  "odd"  or  "accessory" 


190  Edmund  B.    Wilson 

chromosome.  I  still  believe  that  this  explanation  may  be  applic- 
able to  many  cases;  but  a  different  one  seems  more  probable  in  the 
case  of  Metapodius  and  perhaps  may  be  more  widely  applicable. 
This  was  suggested  by  the  observation  (p.  166)  that  in  a  very  few 
cases,  in  22-chromosome  individuals  both  idiochromosomes  were 
seen  passing  to  the  same  pole  in  the  second  division.  The  rare- 
ness of  this  occurrence  shows  that  it  is  doubtless  to  be  regarded  in 
one  sense  as  abnormal.  But  even  a  single  such  event  in  an  original 
22-chromosome  male,  if  the  resulting  spermatozoa  were  functional, 
might  give  the  starting  point  for  the  whole  series  of  relations  ob- 
served in  the  genus,  including  the  establishment  of  an  unpaired  idio- 
chromosome.  The  result  of  such  a  division  should  be  a  pair  of 
spermatozoa  containing  respectively  10  and  12  chromosomes. 
The  former  might  give  rise  at  once  to  a  race  having  an  unpaired 
idiochromosome  and  the  somatic  number  21  in  the  male  (as  in 
Montgomery's  material).  The  latter  might  similarly  produce  an 
individual  having  in  the  first  generation  a  single  supernumerary 
chromosome  and  in  succeeding  generations  an  additional  number. 
This  appears  from  the  following  considerations : 

1  If  a   lo-chromosome  spermatozoon,  arising  in  the  manner 
indicated,  should  fertilize  an  egg  of  the  22-chromosome  class  (hav- 
ing ii  chromosomes  after  reduction)  the  result  should  be  a  male 
containing  21  chromosomes,  the  odd  one  being  the  large  idiochro- 
mosome derived  from  the  egg.     Such  an  individual  would  be  in 
no  respect  distinguishable  from  those  of  Montgomery's  material, 
and  would  similarly  form  male-producing  spermatozoa  containing 
10  chromosomes  and  female-producing  ones  containing  II  (includ- 
ing the  unpaired  idiochromosome).     A  single  such  male,  paired 
with  an  ordinary  22-chromosome  female,  would  suffice  to  establish 
a  stable  race  identical  with  the  form  found  by  Montgomery  at 
West  Chester,  Pa.,  the  males  having  21  chromosomes,  the  females 
having  22,  precisely  as  in  Anasa  or  Leptoglossus.     This  seems  to 
me  the  most   probable  explanation   of  the  conditions   found   in 
Montgomery's  material;  and  possibly  it  may  explain  the  origin 
of  the  unpaired  idiochromosome  in  other  cases  as  well. 

2  The  result  of  fertilizing  the  same  type  of  egg  by  a  spermato- 
zoon from  the  12-chromosome  pole  would  be  an  individual  having 


Studies  on   Chromosomes  191 

23  chromosomes    (egg    n   +  spermatozoon    12)    including    two 
large    idiochromosomes — hence   presumably   a   female — and   one 
small.     The  eggs  produced  by  such  a  female  should  after  matura- 
tion be  of  two  classes,  having  respectively  n  and  12  chromosomes. 
The  12-chromosome  class  would  contain  both  a  large  and  a  small 
idiochromosome,   and   if   fertilized    by  ordinary    n-chromosome 
spermatozoa   would    produce   individuals   with  23  chromosomes, 
male  or  female  according  to  the  class  of  spermatozoon  concerned. 
Such  females  would,  as  before,  contain  two  large  idiochromosomes 
and  one  small.     The  males  would  contain  one  large  and  two  small, 
and  would  accordingly  produce  spermatozoa  having  either  II  or 
12  chromosomes. 

Now,  such  an  additional  small  idiochromosome  in  the  male 
would  be  indistinguishable  from  a  single  "supernumerary  chro- 
mosome" as  it  appears  in  the  23-chromosome  individuals  in  my 
material.  The  resemblance  is  shown  not  only  in  size  but  also  in 
behavior;  for,  as  I  have  shown,  the  supernumerary,  like  the 
idiochromosome,  forms  a  chromosome-nucleolus  during  the  growth 
period,  it  divides  as  a  univalentin  the  first  division,  and  in  the  sec- 
ond is  usually  associated  with  the  idiochromosome  bivalent.  A 
single  such  supernumerary  chromosome,  once  introduced  into  the 
race  would  lead  to  the  presence  of  additional  ones  in  succeeding 
generations.  Thus,  12-chromosome  eggs  fertilized  by  12-chromo- 
some spermatozoa  would  give  individuals  (male  or  female)  with 

24  chromosomes,  including  two  supernumeraries;  and  from  these 
might  arise,  through  irregularities  of  distribution  such  as  I  have 
described,  gametes  with  n,  12,  or  13  chromosomes,  giving  in  the 
next  generation  22,  23,  24,  25  or  26  chromosomes  according   to 
the  particular  combination  established  in  fertilization."     If  this 

11  Since  the  presence  of  an  unpaired  idiochromosome  in  some  individuals  and  of  supernumeraries 
in  others  is  assumed  to  be  traceable  to  the  same  initial  cause,  we  should  naturally  expect  to  find  the  two 
conditions  coexisting  side  by  side,  and  in  approximately  equal  numbers;  but  in  point  of  fact  the  former 
is  very  rare  and  was  only  found  in  one  locality,  while  the  latter  is  very  common.  This  may  constitute 
a  valid  objection  to  my  interpretation.  It  should  be  borne  in  mind,  however,  that  abnormal  divi- 
sions of  the  kind  assumed  to  form  the  starting  point  are  very  rare,  and  that  an  extremely  minute  propor- 
tion of  the  total  number  of  spermatozoa  produced  ever  actually  enter  the  eggs.  The  chances  against 
fertilization  by  either  class  of  the  original  modified  spermatozoa  are  therefore  very  great.  Since  only 
sixty  individuals  have  been  examined  it  need  not  surprise  us  that  one  of  the  two  conditions  in  question 


192  Edmund  B.   Wilson 

interpretation  be  correct,  the  origin  of  an  unpaired  chromosome 
in  certain  individuals  of  this  genus  has  been  owing  to  the  same 
cause  that  has  produced  the  supernumeraries.  Since  both  condi- 
tions coexist  in  the  same  species,  along  with  that  which  may  be 
regarded  as  the  original  type  (22  chromosomes)  it  may  be  conclu- 
ded that  Metapodius  is  now  in  a  period  of  transition  from  the  sec- 
ond to  the  third  of  the  types  distinguished  in  my  last  study.  It 
seems  quite  possible  that  other  species  of  coreids  that  now  have 
constantly  an  unpaired  idiochromosome  may  have  passed  through 
a  similar  condition,  though  in  all  of  them  thus  far  examined  both 
the  small  idiochromosome  and  the  supernumeraries  have  dis- 
appeared. In  Metapodius,  accordingly,  the  supernumeraries 
may  be  regarded  as  on  the  road  to  disappearance.  That  such  is 
the  case  is  rendered  probable  by  the  fact  that  their  number  does 
not  pass  a  certain  limit,  and  is  rarely  more  than  four.  The  very 
small  chromosomes  of  this  kind,  so  often  observed,  are  perhaps 
degenerating,  or  even  vestigial  in  character.  But  aside  from  this, 
attention  has  already  been  called  to  the  probability  that  one  or 
more  of  the  supernumeraries  may  be  lost  during  the  growth-period 
(p.i86);  and  while  this  is  not  certain,  it  may  well  be  that  both 
methods  are  operative  in  their  disappearance. 

The  foregoing  interpretation  of  the  supernumeraries  enables 
us  to  understand  why  variations  in  their  number  are  not  accom- 
panied by  corresponding  morphological  differences  in  the  soma- 
tic characters;  for  they  are  but  duplicates  of  a  chromosome  already 
present  and  hence  introduce  no  new  qualitative  factor.  It  can 
hardly  be  doubted  that  some  kind  of  quantitative  difference  must 
exist  between  individuals  that  show  different  numbers,  but  none 


has  been  more  frequently  met  with.  Another  objection  might  be  based  on  the  different  relations  that 
occur  in  Syromastes.  In  this  form  (see  Wilson  '09)  the  passage  of  both  idiochromosomes  to  one  pole 
without  separation  is  a  normal  and  constant  feature  of  the  second  division,  yet  no  supernumeraries 
appear  in  any  of  the  individuals,  and  it  is  probable  that  the  female  groups  contain  two  pairs  of  idio- 
chromosomes like  the  single  pair  that  appears  in  the  male.  We  have  no  data  for  a  conjecture  as  to  how 
such  a  condition  can  have  arisen;  but  evidently  the  small  idiochromosome  does  not  in  this  case  become 
an  erratic  supernumerary  but  retains  a  definite  adjustment  to  the  other  chromosomes.  Still,  I  do  not 
consider  this  an  obstacle  to  my  interpretation  of  Metapodius,  for  it  is'now  evident  that  the  history  of  the 
idiochromosomes  in  general  has  differed  widely  in  different  species  and  families,  even  among  the  Hemip- 
tera.  We  have  thus  far  only  made  a  beginning  in  their  comparative  study.  [See  Addendum,  p.  200.] 


Studies  on  Chromosomes  193 

has  thus  far  been  discerned.  Such  a  difference  does  not  appear 
in  the  size  of  the  animals,  for  there  are  large  individuals  with  no 
supernumeraries  and  small  individuals  that  possess  them.  An 
interesting  field  for  experiment  seems  here  to  be  offered. 

c     The  "Individuality"  or  Genetic  Continuity  of  the  Chromosomes 

It  is  in  respect  to  this  much  debated  hypothesis  that  the  facts 
observed  in  Metapodius  seem  to  me  most  significant  and  important. 
It  is  evident' that  the  whole  series  of  relations  are  readily  intelligi- 
ble if  the  fundamental  assumption  of  this  hypothesis  be  accepted. 
Without  such  explanation  they  seem  to  me  to  present  an  insoluble 
puzzle.  The  disposition  to  reject  this  hypothesis  that  appears  in  a 
considerable  number  of  recent  papers  on  the  subject  will  doubtless 
lead  to  more  critical  and  exhaustive  observation  of  the  facts;  but 
when  it  goes  so  far  as  to  deny  every  principle  of  genetic  continuity 
in  respect  to  the  chromosomes,  it  is,  I  believe,  a  backward  step. 
This  reaction  perhaps  reaches  a  climax  in  the  elaborate  and 
apparently  destructive  criticism  of  Fick  ('07)  who  considers  the 
hypothesis  to  be  thoroughly  discredited,  and  believes  his  analysis 
to  justify  the  conclusion:  "Dass  weder  theoretisch  noch  sachliche 
Beweise  fur  die  Erhaltungslehre  vorliegen,  sondern  dass  im  Gegen- 
theil  unwiderleghche  Beweise  gegen  sie  vorhanden  smd,  so  dass  es 
im  Interesse  der  Wissenschaft  dringend  zu  wiinschen  ist,  dass 
die  Hypothese  von  alien  Autoren  verlassen  wird"  ('07,  p.  112, 
italics  in  original).  I  incline  to  think  that  this  sweeping  judgment 
would  have  carried  greater  weight  had  Professor  Fick,  in  certain 
parts  of  his  able  and  valuable  discussion,  taken  somewhat  greater 
pains  in  his  presentation  of  facts  and  shown  a  more  judicial  temper 
in  their  analysis.12  To  some  of  the  objections  and  difficulties 

12 1  will  give  two  specific  examples  of  this.  The  experimental  results  of  Moenkhaus  ('04),  on  hybrid 
fishes,  which  evidently  form  a  strong  support  to  the  continuity  hypothesis,  are  unintentionally  but  com- 
pletely misrepresented  in  the  statement  at  p.  75:  "So  berichtet  Moenkhaus  bei  Fundulus-Monidia- 
kreuzung  (sic),  dass  sich  die  beiderlei  (zuerst  sehr  verschiedenartigen)  Chromosomen  in  der  Regel 
schon  nach  der  zweiten  Teilung  nicht  mehr  unterscheiden  lassen."  But  Moenkhaus's  explicit  statement, 
based  on  the  examination  of  "many  thousand  cells,"  is  that  even  in  the  late  cleavage  "Nuclei  showing 
the  two  kinds  of  chromosomes  mingled  together  upon  the  spindle  are  everywhere  to  be  found"  (op.  cit., 
p.  48).  Fick  evidently  had  in  mind  the  fact  that  the  paternal  and  maternal  chromosomes  do  not  as  a 
rule  retain  their  original  grouping  after  the  first  two  or  three  cleavages.  His  actual  statement,  however, 


194  Edmund  B.    Wilson 

brought  forward  in  this  critique  reply  has  already  been  made  by 
Boveri  ('07),  Strasburger  ('08),  Schreiner  ('08)  Bonnevie  ('08)  and 
others.  Some  of  the  difficulties  are  real,  but  an  attentive  study 
of  the  matter  will  show  that  a  large  part  of  Pick's  critique  is 
directed  against  the  strict  hypothesis  of  individuality  and  offers  no 
adequate  interpretation  of  the  essential  phenomenon  that  requires 
explanation.  It  may  be  admitted  that  many  of  the  facts  seem  at 
present  difficult  to  reconcile  with  the  view  that  the  identity  cf  the 
chromosomes  as  actual  individuals  is  maintained  in  the  "resting" 
nucleus;  and  I  have  myself  indicated  (The  Cell,  1900,  p.  300)  that 
the  name  "individuality"  was  perhaps  not  the  best  that  could  have 
been  chosen.  Certainly  we  have  as  yet  comparatively  little  evi- 
dence that  the  chromosomes  retain  their  boundaries  in  the  "rest- 
ing" nucleus.  It  is  evident  that  the  chromosomes  are  greatly 
diffused  in  the  nuclear  network,  and  it  may  be  that  the  substances 
of  different  chromosomes  are  more  or  less  intermingled  at  this  time. 
Pick's  "manoeuvre-hypothesis,"  which  treats  the  chromosomes 
of  the  dividing  cell  as  temporary  "tactic  formations,"  may  there- 
fore be  in  some  respects  a  more  correct  formulation  of  the  facts 
than  that  given  by  the  hypothesis  of  "individuality"  in  the  strict 
sense  of  the  term.  But  the  last  word  on  this  question  has  by  no 
means  yet  been  spoken.  A  new  light  is  thrown  on  it  by  the  recent 
important  work  of  Bonnevie  ('08)  which  brings  forward  strong 
evidence  to  show  that  in  rapidly  dividing  cells  (cleavage  stages 
of  Ascaris,  root-tips  of  Allium),  although  the  identity  of  the  orig- 

(both  here  and  in  the  later  passage  at  p.  98)  will  wholly  mislead  a  reader  not  familiar  with  Moenkhaus' 
work,  in  regard  to  one  of  the  most  significant  and  important  discoveries  in  this  whole  field  of  inquiry. 
Hardly  less  misleading  is  Professor  Pick's  report  of  my  own  observations  on  the  sex-chromosomes 
of  insects,  which  are  stated  as  follows:  "Wilson's  Unterschungen  beweisen  eben  sicker  nur  soviel,  dass 
bei  einigen  Insektengattungen  constante  Beziehungen  zwischen  dem  Geschlecht  und  dem  Vorhandensein 
eines  besonderen  Chromosomenpaares  bestehen,  bei  anderen  Gattungen  nichf  (p.  90).  I  am  confident 
that  those  who  are  familiar  with  the  researches  referred  to  will  not  accept  this  as  a  fair  statement  of  the 
results.  The  fact  is  that  in  one  form  or  other  the  sex-chromosomes  are  present  in  all  of  the  forms  that 
I  have  examined  (now  upwards  of  seventy  species)  and  that  with  various  modifications  all  conform  to  the 
same  fundamental  type.  It  is  true  that  in  two  genera  (Nezara  and  Oncopeltus)  the  sex-chromosomes 
are  equal  in  size,  and  hence  afford  no  visible  differential  between  the  somatic  groups  of  the  two  sexes; 
but  I  especially  emphasized  the  fact  (cf.  '06,  pp.  17,  34)  that  these  chromosomes  are  in  every  other 
respect  identical  with  those  of  other  forms  in  which  the  size-difference  clearly  appears,  and  are  connected 
with  the  latter  by  a  series  of  intermediate  gradations  that  leaves  no  doubt  of  the  essential  uniformity  of 
the  phenomena. 


Studies  on   Chromosomes  195 

inal  chromosomes  is  lost  in  the  "resting"  nucleus  after  each  mito- 
'sis,  each  new  chromosome  nevertheless  arises  by  a  kind  of  endog- 
enous formation  within  and  from  the  substance  of  its  predecessor. 
In  this  way  an  individual  genetic  continuity  of  the  chromosomes 
can  be  directly  followed  through  the  "resting  period"  of  the  nu- 
cleus. "  Eine  genetischeKontinuitat  der  Chromosomen  nacheinan- 
der  folgender  Mitosen  konnte  in  der  von  mir  untersuchten 
Objekten  teils  sicher  (Allium,  Amphiuma)  teils  mit  iiberwiegen- 
der  Wahrsheinlichkeit  (Ascaris)  verfolgt  werden.  Es  ging  aber 
auch  hervor,  dass  eine  Identitdt  der  Chromosomen  verschiedener 
Mitosen  mcht  existtert,  sondern  dass  jedes  Chromosom  in  einem 
fruher  existierenden  endogen  entstanden  ist,  um  wieder  am  Ende 
seines  Lebens  fur  die  endogene  Entstehung  eines  neuen  Chromo- 
soms  die  Grundlage  zu  bilden"  (op.  cit,  p.  54).  Whether  this 
particular  conclusion  will  also  apply  to  more  slowly  dividing  cells 
remains  to  be  seen.  But  apart  from  this  direct  evidence  it  seems 
to  me  that  a  denial  of  every  form  of  genetic  continuity  between 
the  chromosomes  of  successive  cell-generations — which,  despite 
certain  qualifications,  seems  to  be  the  position  of  Fick  and  a  num- 
ber of  other  recent  writers — is  only  possible  to  those  who  are  ready 
to  ignore  some  of  the  most  obvious  and  important  of  the  known 
facts,  especially  those  that  recent  research  has  brought  to  light 
among  the  insects.  The  most  significant  of  these  are : 

1  In  Metapodius  the  specific  number  varies,  while  in  the  indi- 
vidual both  the  number  and  the  size-relations  of  the  chromosomes 
are  constant. 

2  In  all  species  where  the  somatic  chromosome-groups  show 
sexual  differences  in  regard  to  the  number  and  size-relations  of 
the  chromosomes,  exactly  corresponding  differences  exist  between 
the  male-producing  and  the  female-producing  spermatozoa. 

Both  these  series  of  facts  demonstrate  that  the  "tactic  forma- 
tion" of  a  fixed  number  of  chromosomes  of  particular  size  is  not  a 
specific  property  of  a  single  chromatin-substance  as  such,  of  the 
species.  It  has  been  assumed  by  some  writers  that  departures 
from  the  normal  specific  number,  such  as  appear  in  merogonic, 
parthenogenetic,  double-fertilized  or  giant  (double)  eggs,  are  the 
result  merely  of  departures  from  the  normal  quantity  of  chroma- 


196  Edmund  B.   Wilson 

tin."13  If  attentively  considered  the  facts  summarized  above  will, 
I  think,  clearly  show  the  inadequacy  of  such  an  explanation. 
Why  should  a  given  quantity  and  quality  of  chromatin  always 
reappear  in  the  same  morphological  form  as  that  in  which  it 
entered  the  nucleus  ?  Why,  for  example,  in  Metapodius  should 
the  minute  fraction  of  chromatin  represented  by  a  single  small 
supernumerary  always  reappear  in  the  form  of  such  a  chromosome, 
showing  specific  peculiarities  of  behavior,  rather  than  as  a  corre- 
sponding enlargement  of  one  of  the  other  chromosomes  ?  Why 
should  a  larger  excess  always  appear  as  a  group  of  two,  three,  or 
more  supernumeraries  that  differ  definitely  in  behavior  from 
the  others  and  show  constant  size  relations  among  themselves  ? 
Specifically,  in  individual  No.  40,  why  should  two  small  supernu- 
meraries and  two  large  ones  always  appear,  rather  than  three  large 
ones  ?  In  species  where  a  constant  quantitative  chromatin-differ- 
ence  exists  between  the  sexes,  why  should  the  excess  in  the  female 
always  appear  in  the  same  form  as  that  which  appears  in  the  female- 
producing  spermatozoa — in  one  case  as  a  large  idiochromosome 
instead  of  a  small  (Lygaeus),  in  another  as  an  additional  chromo- 
some of  a  particular  size  (very  large  in  Protenor,  small  in  Alydus, 
of  intermediate  size  in  Anasa),  in  a  third  case  as  three  additional 
chromosomes  (Galgulus)  ? 

To  these  and  many  similar  questions  which  the  facts  compel 
us  to  consider,  I  am  unable  to  find  any  answer  on  the  merely 
quantitative  hypothesis.  Each  of  them  receives  a  simple  and 
intelligible  reply  under  the  view  that  it  is  the  number,  size,  and 
quality  of  the  chromosomes  that  enter  the  nucleus  that  determine 
the  number,  size,  and  mode  of  behavior  of  those  that  issue  from 

13  Fick's  treatment  of  these  cases  is  worth  citing.  "Es  muss  von  vornherein  als  wahrscheinlich  be- 
zeichnet  werden,  dass  unter  den  abnormen  Umstanden,  da  einmal  die  Zahl  der'Chromatin-Manoverein- 
heiten'  (im  Sinne  meiner  Manoverhypothese  gesprochen)  in  der  Zelle  erhoht  ist,  diese  Zahl  sich  erhalt" 
(p.  96).  Why  should  the  number  be  maintained  ?  Because,  we  are  told,  "Die  Erhaltung  der  erhb'hten 
Zahl  und  ihre  regelmassige  Wiederkehr  bei  den  folgenden  Teilungungen  muss  bei  dem  nun  einmal 
uber  die  Norm  erhohten  Chromosomenbestand  der  Zelle  als  der  einfachere,  kichter  verstandliche  Far- 
gang  erscheinen,  als  es  ein  besonderer,  ein  "Regulation"  auf  die  Norm  hervorbringender  Akt  ware." 
To  most  readers  this  will  seem  like  an  argument  for,  rather  than  against,  the  hypothesis  of  genetic 
continuity.  But  since  it  is  obviously  not  thus  intended  I  can  discover  no  other  meaning  in  the  passage 
than  that  with  a  given  "bestimmte  Chromatinmanoverart"  characteristic  of  the  species  (p.  115)  the  num- 
ber of  chromosomes  formed  is  proportional  to  the  quantity  of  chromatin-substance. 


Studies  on  Chromosomes  1 97 

it.  But  such  an  answer  implies  the  existence  of  a  definite  indi- 
vidual genetic  relation  between  the  chromosomes  of  successive  cell- 
generations;  and  it  is  this  relation,  I  take  it,  that  forms  the  essence 
of  the  hypothesis  of  genetic  continuity,  whether  or  not  we  include 
in  the  hypothesis  the  assumption  that  the  chromosomes  persist  as 
"individuals"  in  the  resting  nucleus  where  their  boundaries  seem 
to  disappear.  We  might,  for  instance,  assume  that  the  chromo- 
somes are  magazines  of  different  substances  (e.  g.,  enzymes  or  the 
like)  that  differ  more  or  less  in  different  chromosomes,  that  are 
more  or  less  diffused  through  the  nucleus  in  its  vegetative  phase, 
but  are  again  segregated  out  in  the  original  manner  when  the 
chromosomes  reform.14  We  have,  admittedly,  but  an  imperfect 
notion  of  how  such  a  re-segregation  may  be  effected,  though  the 
conclusions  of  Bonnevie  already  referred  to,  constitute  an  impor- 
tant addition  to  the  earlier  ones  of  Boveri  (see  '07,  p.  232)  in  this 
direction.  However  this  may  be,  in  my  view  the  most  practicable, 
indeed  the  almost  necessary,  working  attitude  is  to  treat  the  chromo- 
somes as  if  they  were  actually  persistent  individuals.  The  facts  in 
Metapodius,  which  at  first  sight  seem  to  present  so  chaotic  an 
aspect,  fall  at  once  into  order  and  become  intelligible  if  regarded  as 
due  to  the  presence  in  the  species  of  a  certain  number  of  erratic 
chromosomes,  one  or  more  of  which  may  be  introduced  into  the 
zygote  at  the  timeof  fertilization  and  which  in  some  sense  retain  their 
identity  throughout  the  development.  The  particular  combina- 
tion established  at  the  time  of  fertilization  is  the  result  of  the  chance 
union  of  two  particular  gamete  combinations.  Since  the  distribu- 
tion of  the  supernumeraries  to  the  spermatid  nuclei  is  variable, 
different  gamete  combinations  occur  in  the  spermatozoa  of  the 
same  individual;  and  the  same  is  probably  true  of  the  eggs.  More- 
over, adults  of  the  same  species  live  side  by  side  on  the  same  food- 
plants  and  presumably  may  breed  together.  Different  combinations 
may  thus  be  produced  in  the  offspring  of  a  single  pair,  whether 
the  parents  possess  the  same  or  different  numbers.  Metapodius 
thus  fulfills  the  prediction  of  Boveri,  written  nearly  twenty  years 
ago.  "Wenn  bei  einer  Spezies  einmalsehr  viele  und  verschieden- 

14  A  view  similar  to  this  is  suggested  by  Fick  himself  in  his  earlier  discussion  ('05,  p.  204),   but  it 
does  not  reappear  in  his  later  one. 


198  Edmund  B.    Wilson 

artige  Irregularitaten  vorkamen,  diese  sich  wohl  auf  lange  hinaus 
erhalten  miissten,  so  dass  unter  Umstanden  Falle  mit  ausserordent- 
lich  grosser  Variabilitat  der  Chromosomenzahl  zur  Beobachtung 
kommen  konnten,  ohne  dass  selbst  diese  das  Grundgesetz  umstos- 
sen  vermochten,  welches  lautet:  Es  gehen  aus  jedem  Kerngeriist 
so  viele  Chromosomen  hervor  als  in  die  Bildung  derselben 
eingegangen  sind"  ('90,  p.  61).  To  the  earlier  expression  of 
this  "Grundgesetz"  Boveri  has  recently  added  the  statement 
that  the  chromosomes  that  emerge  from  the  nucleus  are  not  merely 
of  the  same  number  but  also  show  the  same  size-relations  as  those 
that  entered  it.  "Was  durch  den  kurzen  Ausdruck  "Individuali- 
tat  der  Chromosomen"  bezeichnet  werden  soil,  ist  die  Annahme 
dass  fur  jedes  Chromosoma,  das  in  einen  Kern  eingegangen  ist, 
irgend  eine  Art  von  Einheit  im  ruhenden  Kern  erhalt,  welche  der 
Grund  ist,  dass  aus  diesem  ruhenden  Kern  wieder  genau  ebenso 
viele  Chromosomen  hervorgehen  und  dass  dieses  Chromosomen 
iiberdies  da,  wo  vorher  verschiedene  Grossen  unterschieden  waren, 
wieder  in  den  gleichen  Grossenverhaltnissen  auftreten"  ('07,  p. 229). 
The  facts  seen  in  Metapodius  and  other  insects  are  thoroughly 
in  accord  with  the  foregoing  statement,  and  justify  the  additional 
one  that  the  chromosomes  conform  to  the  same  principle  in  respect 
to  their  characteristic  modes  of  behavior.  In  the  Hemiptera 
heteroptera  generally  the  idiochromosomes  and  supernumeraries, 
the  ra-chromosomes,  and  the  "ordinary  chromosomes"  or  "auto- 
somes"  show  each  certain  constant  peculiarities  in  respect  to  the 
time  of  synapsis  and  behavior  during  the  growth-period,  and 
assume  a  characteristic  (though  not  entirely  constant)  mode  of 
grouping  in  the  first  spermatocyte.  Perhaps  the  most  obvious 
of  these  facts  is  the  very  early  condensation  of  the  idiochromo- 
somes and  supernumeraries  in  the  growth-period  as  contrasted  with 
the  other  chromosomes;  and  in  the  case  of  Pyrrochoris  I  have 
shown  ('09)  that  the  idiochromosome  never  assumes  a  diffuse  con- 
dition after  the  last  spermatogonial  division.  But  even  more 
significant  are  the  definite  differences  shown  in  the  couplings  of  the 
various  forms  of  chromosomes  that  take  place  in  the  course  of 
the  spermatogenesis.  Nothing  in  these  phenomena  is  more 
striking  than  the  accuracy  with  which  these  couplings  take  place. 


Studies  on  Chromosomes  199 

As  Montgomery  and  Sutton  have  shown,  the  ordinary  paired 
chromosomes  of  the  spermatogonia  give  rise  to  bivalents  of  corre- 
sponding size  at  the  time  of  general  synapsis.  The  actual  coupling 
of  the  ordinary  chromosomes  at  this  time  is  still  a  matter  of 
dispute;15  but  no  doubt  can  exist  in  regard  to  the  couplings 
that  occur  at  a  later  period  in  case  of  the  ra-chromosomes,  the 
idiochromosomes,  and  the  supernumeraries.  These  characteristic 
couplings  are  not  determined  merely  by  the  size  of  the  chromo- 
somes. The  union  of  the  unequal  idiochromosomes  after  the 
second  division  takes  place  with  the  same  regularity  as  that  of  the 
equal  ra-chromosomes  in  the  prophases  of  the  first.  A  small 
supernumerary  that  is  indistinguishable  from  the  ra-chromosomes 
in  the  spermatogonia  never  couples  with  the  latter  in  either  divi- 
sion, but  with  the  much  larger  idiochromosomes.  The  couplings 
are  equally  independent  of  the  original  positions  of  these  chromo- 
somes, either  in  the  spermatogonia  or  in  the  growth-period,  as  is 
seen  with  especial  clearness  in  case  of  the  m-chromosomes.  These 
phenomena  naturally  suggest  the  conclusion  that  the  couplings 
result  from  definite  affinities  among  the  chromosomes.  The  possi- 
bility no  doubt  exists  that  the  couplings  are  produced  by  extrinsic 
causes  (such  as  the  achromatic  structures)  but  the  evidence  seems 
on  the  whole  opposed  to  such  a  conclusion.  I  consider  it  more 
probable  that  they  are  due  to  intrinsic  qualities  of  the  chromosomes 
and  that  the  differences  of  behavior  shown  by  different  forms  may 
probably  be  regarded  as  due  to  corresponding  physico-chemical 
differences.  This  conclusion  is  in  harmony  with  Boveri's  experi- 
mental results,  though  based  on  wholly  different  data.  While 
it  does  not  seem  worth  while  to  attempt  its  wider  development 
here,  I  may  express  the  opinion  that  all  the  chromosomes  may  con- 
sist in  the  main  of  the  same  material  basis,  differing  only  in  respect 
to  certain  constituents;  and  further  that  the  degree  of  qualitative 
difference  may  vary  widely  in  different  species. 

Zoological  Laboratory 

Columbia  University 

August  10,  1908 

14  See  for  example,  Meves  ('07,  pp.  453-468)  who,  like  O.  Hertwig,  Fick  and  others,  rejects  the  theory 
of  "  individuality." 


20O  Edmund  B.   Wilson 

ADDENDUM 

The  probability  in  regard  to  the  female  groups  of  Syromastes, 
expressed  in  the  footnote  at  p.  192  was  first  stated  in  my  preced- 
ing paper  ('09,  p.  73  )  after  a  study  of  the  male  only.  Since  the 
present  paper  was  sent  to  press  I  have  had  opportunity  to  ex- 
amine females  of  this  form.  The  facts  are  exactly  in  ac- 
cordance with  my  prediction,  the  female  groups  containing  24 
chromosomes,  while  the  male  number  is  22.  It  now  seems  clear, 
however,  that  the  two  idiochromosomes  of  Syromastes  do  not 
correspond  respectively  to  the  large  and  the  small  idiochromo- 
some  of  Metapodius  or  Lygaeus  but  are  equivalent,  taken  together, 
to  the  large  idiochromosome  or  to  the  odd  chromosome  of 
Anasa,  etc. 

October  25, 1908. 

WORKS    REFERRED    TO 

BONNEVIE,  K.  '08 — Chromosomenstudien.     I.     Arch.  f.  Zellforschung,  i,  23. 
BOVERI,  Th.  '90 — Zellenstudien.     III.     Ueber  das  Verhalten  der  chromatischen 

Kernsubstanz  bei  der  Bildung  der  Richtungskorper  und  bei  der  Be- 

fruchtung.     Jena,  1890. 
'07 — Zellenstudien.     VI.     Die  Entwicklung  dispermer  Seeigel-Eier,  etc. 

Jena,  1907. 

FICK,  R.  '05 — Betrachtungen  iiber  die  Chromosomen,  ihre  Individuality,  Reduc- 
tion, und  Vererbung.    Arch.  Anat.  u.  Phys.,  Anat.  Abth.,  Suppl.  1905. 
'07 — Vererbungsfragen,  Reduktions-und   Chromosomenhypothesen,  Bas- 

tard-Regeln.     Merkel  und  Bonnet's  Ergebnisse,  xvi,  1906. 
FOOT,  K.  AND  STROBELL,  E.  C.  '073 — The  "Accessory  Chromosome"  of  Anasa 

tristis.     Biol.  Bull.,  xii. 
'o7b — A  Study  of  Chromosomes  in  the  Spermatogensis  of  Anasa  tristis. 

Am.  Journ.  Anat.,  vii,  2. 
GROSS,  J.  '04 — Die  Spermatogenese  von  Syromastes  marginatus.     Zool.  Jahrb., 

Anat.  Ontog.,  xii. 
LEFEVRE,  G.AND  McGiLL,  C.  '08 — The  Chromosomes  of  Anasa  tristis  and  Anax 

junius.     Am.  Journ.  Anat.,  vii.  4. 
MOENKHAUS,  W.  S.  '04 — The  Development  of  the  Hybrids  between  Fundulus 

heteroclitus  and  Menidia  notata,  etc.     Am.  Journ.  Anat.,  iii. 


Studies  on  Chromosomes  2OI 

MEVES,  Fr. — Die  Spermatocytenteilungen  bei  der  Honigbiene,  etc.     Arch.  Mikr. 

Anat.,  Ixx. 
MONTGOMERY,  T.  H.  '01 — A  Study  of  the  Germ-cells  of  Metazoa.      Trans.  Am. 

Phil.  Soc.,  xx. 
'06 — Chromosomes  in  the  Spermatogenesis  of  the  Hemiptera  Heteroptera. 

Ibid.,  xxi,  3. 
PAULMIER,  F.  C.  '99 — The  Spermatogenesis  of  Anasa  tristis.     Journ.  Morph.,  xv, 

Suppl. 

SCHREINER,  K.  E.  AND  A.  'o8 — Gibt  es  eine  parallele  Konjugation  der  Chromo- 
somen  ?  Videnskebs-Selskabets  Skrifter.  i.  Math-Naturw.  Klasse, 
1908,  no.  4. 

STEVENS,  N.  M.  '06 — Studies  in  Spermatogensis.     II.     A  Comparative  Study  of 
the  Heterochromosomes  in  Certain  Species  of  Coleoptera,  Hemiptera 
and  Lepidoptera,  etc.     Carnegie  Institution.     Pub.  36,  ii. 
'o8a — A  Study  of  the  Germ-cells  of  Certain  Diptera,  etc.     Journ.  Exp. 

Zool.,  v.  iii. 

'o8b — The  Chromosomes  in  Diabrotica  vittata,  etc.     Ibid.,  v,  iv. 
STRASBURGER,  E.  '08 — Chromosomenzahlen,  Plasmastrukturen,Vererbungstrager 

und  Reduktionsteilung.     Jahr.  wiss.  Bot.,  xlv,  iv. 

WILSON,  E.  B.  '053 — Studies  on  Chromosomes,  I.     Journ.  Exp.  Zool.,  ii. 
'o5b — Studies,  etc.     II.     Ibid.,  ii,  iv. 
'06 — Studies,  etc.,  III.     Ibid.,  iii,  I. 
'09 — Studies,  etc.,  IV     Ibid.,  vi,  I. 
'073 — Note  on  the  Chromosome-groups  of  Metapodius  and  Banasa.    Biol. 

Bull.,  xii,  5. 
'07!) — The  Supernumerary  Chromosomes  of  Hemiptera.    Report  of  May 

Meeting.  N.  Y.  Acad.  Sci.  Science,  n.  s.,  xxvi,  677. 
'o7c — The  Case  of  Anasa  tristis.     Science,  n.  s.,  xxv,  631. 


202 


Edmund  B.   Wilson 


APPENDIX 
List  of  individuals  examined,  arranged  according  to  locality 


No. 

Species 

Sex 

Locality 

Supernumeraries 

Somatic 
No. 

^o.  in 
first 
div. 

i 

terminalis 

d1 

Madison,  N.  J.  (Paulmier) 

I  small 

23 

13 

2 

terminalis 

ff 

Madison,  N.  J.  (Paulmier) 

I  small 

23 

13 

3-1  1 

terminalis 

d* 

West  Chester,  Pa.  (Montgomery) 

absent 

21 

n 

12 

terminalis 

if 

West  Chester,  Pa.  (Wilson) 

absent 

22 

12 

'3 

terminalis 

d1 

West  Chester,  Pa.  (Wilson) 

i  large 

23 

!3 

14 

terminalis 

9 

West  Chester,  Pa.  (Wilson) 

i  large 

23 

15 

terminalis 

9 

West  Chester,  Pa.  (Wilson) 

2  large 

24 

16 

terminalis 

9 

West  Chester,  Pa.  (Wilson) 

2  large 

24 

i? 

terminalis 

cT 

Mansfield,  Ohio 

absent 

22 

12 

if 

terminalis 

9 

Mansfield,  Ohio 

absent 

22 

»9 

terminalis 

d1 

Raleigh,  N.  C. 

absent 

22 

12 

20 

terminalis 

d1 

Raleigh,  N.  C. 

i  large 

23 

13 

21 

terminalis 

(f 

Raleigh,  N.C. 

2  large 

24 

14 

22 

terminalis 

* 

Raleigh,  N.  C. 

I  large,  I  small 

24 

H 

23 

terminalis 

9 

Raleigh,  N.  C. 

absent 

22 

24 

terminalis 

9 

Raleigh,  N.  C. 

absent 

22 

2S 

terminalis 

9 

Raleigh,  N.  C. 

i  large 

23 

26 

terminalis 

9 

Raleigh,  N.  C. 

2  large 

24 

2? 

terminalis 

9 

Raleigh,  N.  C. 

2  large,  I  small 

25 

28 

femoratus 

d1 

Raleigh,  N.  C. 

absent 

22 

12 

29 

femoratus 

d1 

Raleigh,  N.  C. 

absent 

22 

12 

3° 

femoratus 

9 

Raleigh,  N.  C. 

f  large 

23 

31 

femoratus 

9 

Raleigh,  N.  C. 

2  large 

24 

32 

femoratus 

9 

Raleigh,  N.  C. 

4  large 

26 

33 

femoratus 

9 

Raleigh,  N.  C. 

3  larSe 

2-3    small 

27-8 

34 

terminalis 

c? 

Southern  Pines,  N.  C. 

3  lar§e 

25 

15 

35 

terminalis 

d1 

Southern  Pines,  N.  C. 

3  large 

2S 

IS 

36 

terminalis 

d1 

Southern  Pines,  N.  C. 

4  lar§e 

26 

ft 

37 

terminalis 

c? 

Southern  Pines,  N.  C. 

2  large 

24 

H 

38 

terminalis 

d1 

Southern  Pines,  N.  C. 

3  large 

25 

39 

femoratus 

d1 

Southern  Pines,  N.  C. 

2  large 

24 

H 

40 

femoratus 

d1 

Southern  Pines,  N.  C. 

2  large,  2  small 

26 

16 

4i 

femoratus 

9 

Southern  Pines,  N.  C. 

i  large 

n 

42 

femoratus 

d1 

Columbia,  S.  C. 

4  large 

26 

16 

43 

terminalis 

d1 

Charleston,  S.  C. 

i  small 

23 

J3 

44 

terminalis 

9 

Charleston,  S.  C. 

absent 

22 

Studies  on   Chromosomes 


203 


List  of  individuals  examined,  arranged  according  to  locality— Continued 


No. 

Species 

Sex 

Locality 

Supernumaries 

Somatic 
No. 

No.  in 
first 
div. 

45 

femoratus 

9 

Charleston,  S.  C. 

2  large                          24 

46 

femoratus 

d" 

Savannah,  Ga. 

absent 

22 

12 

47 

granulosus 

c? 

Tucson,  Arizona 

absent 

22 

12 

48 

granulosus 

<? 

Tucson,  Arizona 

I  large 

23 

'3 

49 

granulosus 

d1 

Tucson,  Arizona 

i  large 

23 

!3 

5° 

granulosus 

c? 

Tucson,  Arizona 

2  large 

24 

H 

5' 

granulosus 

d1 

Tucson,  Arizona 

2  large 

M 

H 

5* 

granulosus 

<? 

Tucson,  Arizona 

2  large 

24 

'4 

S3 

granulosus 

d1 

Tucson,  Arizona 

2  large 

(H) 

14 

54 

granulosus 

d1 

Tucson,  Arizona 

3-4  large 

25-26 

15-16 

55 

granulosus 

d1 

Tucson,  Arizona 

4  large 

26 

16 

56 

granulosus 

d1 

Tucson,  Arizona 

4  large 

26 

16 

57 

granulosus 

d1 

Tucson,  Arizona 

4  large,  I  small 

(»T) 

i? 

58 

granulosus 

9 

Tucson,  Arizona 

3  large 

25 

59 

granulosus 

c? 

Grand  Canyon,  Arizona 

4  large 

26 

16 

60 

granulosus 

d1 

Grand  Canyon,  Arizona 

4  large 

(26) 

16 

61 

granulosus 

9 

Grand  Canyon,  Arizona 

4  krge 

26 

62 

granulosus 

9 

Grand  Canyon,  Arizona 

±  4  krge 

±26 

204  Edmund  B.   Wilson 

EXPLANATION  OF  PLATE  I. 

The  figures  are  reproduced  directly  from  the  original  photographs,  without  retouching,  at  an  enlarge- 
ment of  1500  diameters.  It  should  be  borne  in  mind  that  in  the  photographs  considerable  apparent  size- 
variations  are  produced  by  differences  of  focus,  and  that  unless  the  chromosomes  lie  exactly  in  one  plane 
the  photograph  often  gives  a  less  accurate  impression  than  a  drawing.  Drawings  of  most  of  these  photo- 
graphs with  designations,  will  be  found  among  the  text  figures,  as  indicated. 

1  M.  terminalis  (No.  3,  Montogmery's  material),  2i-chromosome  form,  first  spermatocyte-division 
polar  view;  unpaired  idiochromosome  (odd  or  accessory)  outside  the  ring,  to  the  right  (Fig.  3,  i). 

2  M.  terminalis  (No.  19),  22-chromosome  form,  first  division,  polar  view;  the  two  separate  idio- 
chromosomes  at  the  right.     (The  small  idiochromosome,  being  slightly  out  of  focus,  appears  too  small. 
Its  size  is  correctly  shown  in  the  drawing,  Fig.  4,  b\ 

3  M.  terminalis  (No.   12),  22-chromosome  form   similar  view;   idiochromosomes    in  contact 

(Fig-4,  /)• 

4  M.  terminalis  (No.  20),  23-chromosome  form,  one  large  supernumerary,  view  similar  to  the  pre- 
ceding; idiochromosomes  and  supernumerary  to  the  right  (Fig.  i,  g). 

5  M.  granulous  (No.  49),  23-chromosome  form,  one  large  supernumerary,  which  lies  inside  the 
ring  with  the  small  idiochromosome  and  m-chromosome  (Fig.  7,  g). 

6  M.  terminalis  (No.  i),  23-chromosome  form,  one  small  supernumerary  lying  inside  the  ring 
with  the  w-chromosome  and  one  of  the  large  bivalents  (Fig.  7,  /'). 

7  M.  granulosus  (No.  52),  24-chromosome  form,  two  large  supernumeraries  (Fig.  n,  g). 

8  M.  femoratus  (No.  42),  26-chromosome  form,  four  large  supernumeraries  (Fig.  2,  g). 

9  M.  terminalis  (No.  36),  26-chromosome  form,  similar  to  preceding  (Fig.  9,  e). 

10  M.  femoratus  (No.  57),  27-chromosome  form,  four  large  supernumeraries  and  one  small  (Fig. 
13,  h). 

1 1  M.  femoratus  (No.  46),  22-chromosome  form,  first  division  in  side  view,  both  idiochromosomes 
dividing  (Fig.  4,  /'). 

12  M.  granulosus  (No.  47)  22-chromosome  form,  second  division,  polar  view  (Fig.  5,  c). 

13  M.  femoratus  (No.  42),  26-chromosome  form;  second  division,  polar  view,  showing  hexad  ele- 
ment near  center  (Fig.  10,  a). 

14  M.  terminalis  (No.  3,  Montgomery's  material)  2i-chromosome  form,  second  division  side  view, 
showing  lagging  idiochromosome  ("accessory  chromosome")  (Fig.  3,  /). 

15  From  the  same  cyst  as  the  last,  later  stage  of  second  division;  idiochromosome  entering  one  pole 

(Kg.  3»  g)- 

1 6  M.  femoratus  (No.  29),  22-chromosome  form,  second  division  metaphase  in  side  view,  showing 
idiochromosome  bivalent  (like  Fig.  5,  d). 

17  M.  granulosus  (No.  47),  22-chromosome  form,  late  anaphase  of  second  division,  one  idiochromo- 
some entering  each  pole  (Fig.  5,  /). 

18  M.  femoratus  (No.  46),  abnormal  late  anaphase  of  second  division,  showing  both  idiochromo- 
somes passing  to  the  same  pole  (Fig.  5,  o). 

19  M.  femoratus  (No.  29),  22-chromosome  form,  second  division  showing  initial  separation  of  the 
idiochromosomes  (like  Fig.  5,  /). 

20  M.  granulosus  (No.  49),  23-chromosome  form,  one  large  supernumerary,  second  division  meta- 
phase, showing  triad  element  formed  by  the  union  of  the  supernumerary  with  the  idiochromosome- 
bivalent  (like  Fig.  8,  »'). 

21  M.  granulosus  (No.  52),  24-chromosome  form,  two  large  supernumeraries,  second  division,  show, 
ing  tetrad  element  consisting  of  the  idiochromosomes  and  supernumeraries  united  in  a  linear  series  (Fig. 
II,  «). 


Studies  on  Chromosomes  205 

22  M.  femoratus  (No.  42),  26-chromosome  form,  four  large  supernumeraries;  second  division  show- 
ing hexad  element  formed  by  the  idiochromosomes  and  supernumeraries  (Fig.  10,  h). 

23  From  the  same  cyst,  similar  view  (Fig.  10,  k~). 

24  M.  terminalis  (No.  3,  Montgomery's  material),  2i-chromosome  form,  nucleus  from  the  growth- 
period,  showing  single  spheroidal  chromosome  nucleolus  (like  Fig.  3,  /). 

25  M.  femoratus  (No.  29),  22-chromosome  form,  growth-period,  showing  double  chromosome- 
nucleolus  (idiochromosome-bivalent)  and  plasmosome  (Fig.  6,  6). 

26  From  the  same  slide,  showing  different  ordinary  chromosomes,  separate  chromosome-nucleoli 
and  plasmosome  (Fig.  6,  c). 

27  M.  terminalis  (No.  20),  ^-chromosome  form,  growth-period,  showing  tripartite  chromosome- 
nucleolus  formed  by  the  idiochromosomes  and  supernumerary  (like  Fig.  i,  /). 

28  M.  granulosus  (No.  60),  26-chromosome  form,  growth-period,  showing  hexad  chromosome- 
nucleoli  from  three  different  cells  (like  Fig.  10,  j-«). 

29  M.  terminalis  (No.  2),  23-chromosome  form,  one  small  supernumary;  spermatogonial  group, 
showing  three  small  chromosomes  (the  supernumerary  and  two  m-chromosomes);  the  small  idiochromo- 
some  distinguishable  above  towards  the  left  (Fig.  7,  y). 

30  M.  terminalis  (No.  22),  24-chromosome  form,  one  small  supernumerary  and  one  large  (Fig.  1 1,  p) 


LJDItS     ON     CHROMOSOMES     V. 

(E.   B    Wilson) 


PLATE   I. 


*.. 


•• 


•** 


'•>.- 


* 


12 


13 


16 


17 


.     .     29 

•  •      •  • 

The  Journal  of  Experimental  Zoology,  Vol.  VI.  .  •. 


STUDIES  ON  CHROMOSOMES 


VI.      A  NEW  TYPE  OF  CHROMOSOME  COMBINATION 
IN  METAPODIUS 


RETURN  TO 

DIVISION  OF  GENETICS 

HILGARO  HALL 


Ki)\ir\i)  H.  WILSON 

WITH    FI\  I 


REPRINTED    FKOM 

THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY 

Volume  IX        No.  1 


\Vn,!,IAMS  .fe  SVILKIXS  COMI'ANV 
BALTIMORE 


Reprinted  from  THE  JOURNAL  OF  EXPEIKMENTAL  ZOOLOGY  VOL.  9,  No.  1. 


STUDIES  ON  CHROMOSOMES 

VI.       A  NEW  TYPE  OF  CHROMOSOME   COMBINATION  IN  METAPODITJS 

EDMUND    B.    WILSON 

Professor  of  Zoology,  Columbia  University. 

WITH   FIVE   FIGURES 

Although  the  peculiar  combination  of  chromosomes  here  to  be 
described  has  been  seen  in  only  a  single  individual,  it  affords  new 
and  I  think  significant  evidence  regarding  some  of  the  most  inter- 
esting of  the  problems  connected  with  the  nuclear  organization. 
As  was  shown  in  the  fifth  of  my  "Studies  on  Chromosomes,"1 
the  genus  Metapodius  is  most  exceptional  and  remarkable  in 
that  the  specific  number  of  chromosomes  varies,  while  that  of  the 
individual  is  on  the  whole  constant.  It  is  true  that  slight  indis- 
criminate fluctuations  in  the  number  of  the  ordinary  chromosomes, 
or  "autosomes, "  occur,  as  they  do  in  many  other  species;  but 
this  is  only  an  inconsiderable  source  of  the  specific  variation.  The 
evidence  shows,  beyond  a  doubt  in  some  individuals,  and  hence 
with  probability  for  all,  that  the  numerical  differences  are  pri- 
marily due  to  variations  in  the  number  of  a  particular  class  of 
chromosomes  which  I  called  the  "supernumeraries."  These 
may  be  wholly  absent.  When  present,  their  number  is  constant 
in  the  individual,  but  differs  in  different  individuals.  They  are 
often  recognizable  in  both  sexes  by  their  size,  and  in  the  male 
also  by  certain  very  definite  peculiarities  of  behavior  in  the  matu- 
ration-process. When  they  are  absent,  the  diploid  groups  contain 
22  chromosomes;  and  this  condition  is  almost  certainly  the  funda- 
mental type  of  the  genus,  of  which  all  the  other  conditions  are 
variants.  Such  a  group  comprises  18  ordinary  chromosomes,  or 
"autosomes"  +  2  very  small  microchromosomes,  or  m-chrom- 
somes  +  2  unequal  idiochromosomes  =  22  (these  respective 

1  Wilson:  '09c. 

THE  JOURNAL  OP   EXPERIMENTAL  ZOOLOGY     VOL.   9,    NO.    1. 


54  EDMUND    B.    WILSON 

classes  having  the  peculiarities  heretofore  described).2  Numbers 
above  22  arise  through  the  addition  of  one  or  more  relatively 
small  "supernumeraries,"  which  agree  in  behavior  with  the  small 
idiochromosome,  of  which  they  are  probably  duplicates.  None 
of  my  own  material  (53  individuals,  of  three  species)  showed  less 
than  22  chromosomes,  and  at  least  one  small  idiochromosome  was 
present  in  all.  In  all  of  Montgomery's  material  of  M.  terminalis, 
however  (9  individuals),  this  chromosome  is  absent,  the  sperma- 
togonial  number  is  but  21,  and  the  large  idiochromosome  appears 
without  a  synaptic  mate  as  a  typical  odd  or  accessory  chromosome. 
The  foregoing  results  were  based  on  the  study  of  62  individuals 
in  all,  representing  the  three  species,  terminalis,  femoratus  and 
granulosus.  In  February,  1909, 1  took  at  Miami,  Fla.,  two  addi- 
tional male  specimens  of  femoratus,  quite  typical  in  structure, 
and  closely  similar  in  external  appearance.  One  of  these  (No.  63) 
is  an  ordinary  23-chromosome  form  with  one  large  supernumerary 
(like  Nos.  13  or  48  of  the  general  list  given  in  "  Study  V")  and  is 
only  of  interest  for  comparison  with  the  other  individual.  The 
latter,  hereinafter  designated  as  "No.  64,"  shows  a  different 
chromosome-combination  from  any  heretofore  seen  in  this  genus 
or  elsewhere.  The  diploid  groups  (spermatogonia)  contain  22 
chromosomes;  but  both  these  groups  in  themselves  and  their 
history  in  maturation  proves  most  clearly  that  they  are  not  the 
same  as  in  the  typical  22-chromosome  forms,  differing  from  the 
latter  in  respect  to  the  idiochromosom.es  and  the  m-chromosomes. 
In  the  typical  forms  there  are,  as  stated  above,  two  of  each  of 
these  chromosomes.  In  No.  64,  on  the  other  hand,  there  are  three 
m-chromosomes  and  but  one  idiochromosome  (the  large),  the 
latter  appearing  as  a  typical  odd  or  accessory  chromosome,  as  in 
the  material  of  Montgomery;  thus,  18  autosomes  +  3  m-chromo- 
somes +  1  odd  chromosome  =  22.  That  this  is  the  true  interpre- 
tation of  the  facts  is  demonstrated  by  the  behavior  of  these  respec- 
tive chromosomes  in  the  maturation-process.  I  would  emphasize 
the  fortunate  fact  that  both  testes  of  the  animal  show  excellent 
fixation  and  staining  (strong  Flemming,  iron  haematoxylin)  and 
that  they  contain  multitudes  of  division-figures  which  demonstrate 

*  Ibid: '056,  05c,  '06,  etc. 


STUDIES   ON   CHROMOSOMES  56 

all  the  stages.  The  agreement  of  great  numbers  of  division-fig-- 
ures  from  both  testes  leaves  no  doubt  regarding  the  constancy  of 
the  essential  phenomena  (with  rare  minor  variations,  as  indicated 
beyond).  As  will  be  seen,  the  modification  of  the  diploid  groups 
has  led  to  corresponding  modifications  of  the  maturation-process 
that  are  most  interesting  in  relation  to  some  of  the  problems  of 
synapsis  and  of  the  qualitative  differences  of  the  chromosomes. 

DESCRIPTIVE 
a.     The  spermatogonial  groups 

The  peculiar  anomaly  of  the  chromosome  groups,  first  seen  in 
the  spermatocyte-divisions,  led  me  to  examine  the  spermatogonial 
groups  with  particular  care,  and  it  will  be  worth  while  to  state 
both  the  preliminary  and  the  definitive  results.  These  groups 
are  in  the  nature  of  the  case  more  difficult  than  those  of  the  sper- 
matocytes,  owing  to  the  greater  number,  smaller  size,  and  greater 
crowding  of  the  chromosomes;  hence,  only  flat  metaphase-plates 
and  such  as  are  not  very  oblique  to  the  plane  of  section  can  safety 
be  used.  A  search  through  the  numerous  dividing  spermatogonia 
showed  35  cases  that  seemed  to  meet  these  conditions  and  also 
to  show  no  serious  obscurity  or  confusion  of  the  chromosomes. 
Many  of  these  are  of  almost  schematic  clearness,  and  some  are 
well  adapted  for  photographic  reproduction.  The  first  examina- 
tion showed  undoubtedly  that  29  of  the  35  cases  contained  22 
chromosomes  each,  including  19  large  and  three  very  small  ones. 
Of  the  six  exceptions,  three  seemed  to  lack  one  of  the  small  ones, 
two,  one  of  the  large  ones,  and  one  a  large  and  a  small.  Closer 
study  of  these  six  cases  ultimately  showed  that  in  four  cases 
the  apparently  missing  third  small  chromosome  was  in  reality 
present,  though  hidden  among  the  larger  chromosomes,  while  in 
two  cases  an  apparently  missing  larger  chromosome  was  found 
lying  immediately  below  another  one,  the  metaphase-plate  not 
yet  having  become  perfectly  flat.  This  leaves  but  one  exception 
in  35  cases,  and  we  shall  hardly  go  astray  in  the  conclusion  that 
this  exception  is  probably  the  result  of  accident.  In  any  case  we 
may  confidently  conclude  that  the  chromosome-group  shown  in 


56  EDMUND   B.   WILSON 

the  34  cases  may  be  taken  as  characteristic  of  the  dividing  sper- 
matogonia,  and  that  it  occurs  with  a  high  degree  of  constancy. 

Six  of  these  groups,  from  the  best  that  could  be  found,  three 
from  each  testis,  are  shown  in  fig.  i,  a-/.  These  have  been  se- 
lected particularly  to  show  the  different  positions  of  the  three 
small  chromosomes.  The  latter  appear  to  follow  no  rule  what- 
ever, the  three  lying  anywhere  in  the  metaphase-plate;  and  all 
may  be  separate,  all  together,  or  two  together  and  one  separate. 
This  is  an  interesting  and  significant  fact,  because  in  the  first 
spermatocyte-division,  as  described  beyond,  the  three  are  always 
associated  to  form  a  triad  element  which  invariably  occupies  the 
same  position  in  the  chromosome-group  (see  p.  58). 

For  the  sake  of  comparison,  four  sperm atogonial  groups  from 
other  individuals  are  here  reproduced  (from  my  fifth  Study). 
Two  of  these  (fig.  i,  i,  j]  are  from  femoratus,  No.  29,  which  has 
the  typical  diploid  group  of  22  chromosomes,  including  but  two 
small  (m-chromosomes.)  The  other  two  (fig.  i,  g,  h)  are  from 
terminalis,  No.  2,  which  has  23  chromosomes,  including  two  m- 
chromosomes  and  one  small  supernumerary3.  As  will  appear 
beyond,  this  latter  chromosome  is  wholly  different  in  nature  from 
the  third  small  chromosome  in  individual  No.  64,  though  indis- 
tinguishable from  it  by  the  eye  in  the  spermatogonial  groups. 

As  the  figures  show,  the  larger  chromosomes  in  No.  64  show 
well  marked  size-differences,  and  in  most  of  the  groups  a  largest 
and  second  largest  pair  are  usually  fairly  evident;  but  it  is  impos- 
sible to  pair  all  of  the  chromosomes  accurately  by  the  eye.  It 
is,  however,  obvious  that  not  more  than  18  of  the  19  can  be  equally 
paired.  One  of  them  must  either  have  no  proper  mate,  or  it  must 
form  a  very  unequal  pair  with  the  third  small  chromosome.  The 
following  possibilities  must,  accordingly,  be  considered: 

1.  The  nineteenth  large  chromosome  and  the  third  small  one 
are  respectively  a  large  and  an  abnormally  small  idiochromosome 
which  form  a  pair  of  synaptic  mates,  or 

2.  The  nineteenth  large  chromosome  is  an  odd  or  accessory 
chromosome,  without  a  synaptic  mate,  while  the  third  small  one 
is  similar  to  a  small  "supernumerary"  or 

*  Cf:  Photo.  29,  Study  V. 


STUDIES   ON   CHROMOSOMES 


57 


All  the  figures  are  from  camera  lucida  drawings.    With  a  few  exceptions  they 
are  a  little  more  enlarged  than  those  of  Study  V. 


o 


Fig.  1  a-f,  spermatogonial  groups,  M.  femoratus,  No.  64,  three  from  each  tes- 
tis;  g,  h,  spermatogonial  groups  for  comparison  from  M.  terminalis,  No.  2, 
with  one  small  supernumerary  (23  chromosomes);  i,  j,  spermatogonial  groups 
from  M.  femoratus,  No.  29  (22  chromosomes);  k,  I,  early  prophases,  No.  64;  m, 
n,  late  prophase  of  same ;  o,  late  prophase  for  comparison,  from  M.  terminalis,  No. 
43,  with  one  small  supernumerary  (23  chromosomes). 


58  EDMUND    B.    WILSON 

3.  An  odd  or  accessory  chromosome  is  present,  and  also  a 
third  w-chromosome. 

A  study  of  the  maturation  process  decisively  establishes  the 
third  of  these  possibilities  as  the  fact. 

b.     The  first  spermatocyte-division 

Both  testes  contain  immense  numbers  of  both  spermatocyte- 
divisions  in  all  stages,  and  many  of  the  cysts  show  the  facts  with 
great  beauty.  The  first  division  itself  at  once  indicates  the  true 
interpretation  of  the  spermatogonial  groups;  and  this  is  consis- 
ently  borne  out  by  the  stages  which  precede  and  follow. 

In  polar  views  (fig.  2,  d-g]  the  first  division  metaphase  is  iden- 
tical in  appearance  with  that  of  Anasa,  Chelinidea,  Narnia,  and 
other  coreids  that  have  21  spermatogonial  chromosomes  (including 
Montgomery's  individuals  of  M.  terminalis).  Eleven  chromo- 
somes appear,  including  one  very  small  central  one  surrounded 
by  a  ring  of  nine  much  larger  ones,  while  the  eleventh  usually 
occupies  a  position  outside  the  ring,  as  in  figs.  2d,  2f,  (figs.  2e,  2g 
are  given  to  show  exceptions  to  this).  From  these  views  alone 
we  should  infer  that  the  spermatogonial  number  is  21,  that  the 
small  central  chromosome  is  the  m-bivalent,  and  that  the  eccentric 
one  is  the  accessory.  This  will  appear  upon  comparison  with 
figs.  2,  h,  i,  which  show  two  corresponding  views  of  Montgomery's 
material  of  M.  terminalis.  Side  views  at  once  reveal  the  fact, 
however,  that  the  central  body  in  No.  64  is  not  a  bivalent  but  a 
triad  element,  consisting  of  three  small  chromosomes  united  end 
to  end  (figs.  2b,  3a,  &,)  and  it  is  perfectly  evident  that  these  are 
identical  with  the  three  very  small  ones  of  the  spermatogonial 
groups.  Hundreds  of  these  figures  have  been  observed,  iri  almost 
all  of  which  the  three  components  have  the  linear  arrangement 
just  described;  but  now  and  then  a  different  grouping  occurs,  as 
may  be  seen  in  both  side  (fig.  3c)  and  polar  views  (fig.  2g). 

In  the  ensuing  division  the  ten  larger  chromosomes  divide 
equally,  showing  as  they  draw  apart  the  curious  forms  represented 
in  fig.  4,  which  are  closely  similar  to  those  described  in  Anasa 
by  Paulmier  ('99).  As  the  figures  show,  the  chromosomes  in 


STUDIES   ON   CHROMOSOMES 


59 


d 


4J*  A 

%•  *  w  *-*• ,  * 


'• 

>6 


I 


••; 

>i' 


Fig.  2  a,  late  prophaseNo.  64,  spindle  forming;  b,  metaphase  of  same  in  side 
view;  c,  late  prophase  of  M.  femoratus,  No.  29  (22  chromosomes),  for  comparison 
with  a;  d-g,  first  division  metaphase,  polar  views,  No.  64;  h,  i,  similar  views  of 
M.  terminalis,  No.  3  (Montgomery's  material),  with  21  chromosomes;./,  similar 
view  of  M.  femoratus,  No.  29  (22  chromosomes);  k,  similar  view  of  M.  terminalis, 
No.  43  (23  chromosomes),  with  one  small  supernumerary,  for  comparison. 

the  early  anaphase  are  more  or  less  clearly  quadripartite,  and  sep- 
arate into  bipartite  daughter  chromosomes  connected  by  conspic- 
uous double  fibers  (figs.  4,  b-h)  but  true  tetrads  (such  for  instance, 
as  those  observed  by  Levfere  and  McGill  in  Anax,  '08)  are  rarely 
if  ever  seen.  The  quadripartite  form,  though  very  characteristic 
of  this  division,  is  by  no  means  invariable  in  case  of  the  large 
bivalents,  and  has  not  been  seen  in  case  of  the  eccentric  odd 
chromosome. 


60  EDMUND    B.    WILSON 

In  the  mean  time  the  small  central  triad  breaks  up  into  its 
separate  components,  which  then  pass  to  the  poles  in  a  very  inter- 
esting fashion.  This  process  always  begins  before  the  division  of 
the  large  chromosomes,  and  is  subject  to  some  variation.  Most 
frequently  the  three  components  draw  apart  in  such  a  way  as  to 
leave  the  middle  one  lagging  near  the  equator  of  the  spindle  while 
the  others  are  proceeding  towards  the  poles  (flgs.  3/i,  i).  Often, 
however,  one  component  first  separates  from  the  other  two  (figs. 
3j,  k) ;  but  even  in  this  case  it  seems  probable  that  one  of  the  latter 
is  afterwards  left  lagging  on  the  spindle,  since  later  in  the  anaphases 
this  arrangement  is  almost  invariable.  In  these  stages  the  middle 
component  frequently  becomes  drawn  out  along  the  spindle  to 
form  a  rod  which  finally  passes  to  one  pole  to  enter  the  telophase 
group  (figs.  4e,/) .  Half  the  secondary  spermatocy  tes  thus  receive 
two  small  chromosomes  and  half  but  one,  the  respective  numbers 
being  12  and  11. 

Two  observed  anomalies  may  briefly  be  mentioned.  In  two  or 
three  cases  the  middle  component  seems  to  be  degenerating  on 
the  spindle  (fig.  40) ;  but  if  this  be  really  the  case  it  must  be  of 
rare  occurrence,  as  is  shown  by  the  second  division.  Another 
interesting  anomaly  is  shown  in  fig.  4/i.  Here  there  are  appar- 
ently five  small  chromosomes,  two  of  which  are  smaller  than  the 
others  and  are  connected  by  a  fiber  as  if  they  had  recently  divided. 
I  am  uncertain  how  to  interpret  this  case,  for  one  of  the  larger 
chromosomes  (stippled  in  the  figure)  is  paler  than  the  others  and 
lies  at  a  lower  level.  This  may  be  a  fragment  of  the  original 
plasmosome.  If  this  be  the  case  we  have  before  us  a  case  in  which 
the  central  small  chromosome  has  divided  precociously.  If  all 
the  five  bodies,  on  the  other  hand  be  chromosomes,  one  of  them 
would  seem  to  be  an  extra  or  adventitious  body,  comparable  to 
those  described  and  figured  by  Paulmier  in  Anasa  ('99,  fig.  28a). 

c.     The  second  spermatocy te-division 

As  is  to  be  expected  from  the  asymmetrical  distribution  of  the 
three  small  chromosomes  in  the  first  division  the  secondary  sper- 
matocvtes  are  of  two  classes.  These  divisions  are  very  numerous 


STUDIES    ON    CHROMOSOMES 


61 


Fig.  3  Metaphases  and  anaphases  of  the  first  division,  No.  64,  in  side  view,  a, 
b,  typical  side  views,  with  linear  central  m-triad;  c,  unusual  grouping;  d,  similar 
view  of  M.  terminalis,  No.  1,  for  comparison,  with  one  small  supernumerary  and 
m-bivalent  (23  chromosomes) ;  e.  g,  similar  views  of  M.  femoratus,  No.  29  (22  chro- 
mosomes) for  comparison;/,  the  same,  M.  femoratus,  No.  46  (22 chromosomes) ; 
h-k,  No.  64,  initial  anaphase,  separation  of  the  m-triad. 

in  both  testes,  and  all  the  stages  are  shown  by  hundreds.  In 
polar  views  of  the  metaphases  about  half  the  cells  are  seen  to 
contain  11  chromosomes  (fig.  5a,  6)  and  half  12  (fig.  5c,  d),  the 
former  containing  but  one  small  chromosome  and  the  latter  two. 


62 


EDMUND   B.   WILSON 


Fig.  4    Anaphases  of  first  division,  all  from  No.  64;  g  and  h  are  atypical  condi- 
tions. 


As  is  the  rule  throughout  the  Coreidse,  the  regular  grouping  char- 
acteristic of  the  first  division  is  usually  lost  or  obscured  in  the 
second.  As  a  rule  the  ring  formation  is  no  longer  seen,  there  is, 
no  constantly  eccentric  chromosome,  while  the  w-chromosome, 
invariably  central  in  position  in  the  first  division,  now  occupies 
any  position,  though*  it  is  more  frequently  near  the  center  of  the 
group. 

In  side  views  of  the  metaphases  all  of  these  chromosomes,  with 
one  important  exception,  are  dumb-bell  shaped,  and  in  the  initial 
anaphases  are  seen  drawing  apart  into  a  pair  of  daughter-chromo- 
somes (fig.  5e-gr).  One  chromosome,  almost  invariably  central 
in  position,  forms  an  exception  in  showing  no  sign  of  constriction, 
its  form  being  evenly  rounded  and  often  nearly  spheroidal.  As 


•  »• 


•  •  a 


fST  \ 

,  m 

I  m 


p 


III! 


Fig.  5  Second  division,  No.  64.  a,  6,  metaphases,  polar  views,  11  chromo- 
somes; c,  d,  the  same,  12  chromosomes;  e,  side  view;  /,  the  same  showing  all  the 
chromosomes  (four  from  lower  level  shown  below) ;  g,  initial  anaphase,  all  the 
chromosomes  shown  (five  of  them  from  a  lower  level  at  right) ;  h-o,  later  anaphases; 
p,  q,  sister  groups,  from  the  same  spindle,  late  anaphase,  p,  the  upper  group  with 
11  chromosomes;  q,  the  lower  group  with  12;  r,  s,  two  late  anaphase  groups  (not 
from  the  same  spindle)  to  show  the  third  and  fourth  types  of  spermatid  nuclei. 


64  EDMUND    B.    WILSON 

seen  in  side  views  of  the  late  metaphases  or  earliest  anaphases 
(fig.  50)  this  chromosome  always  appears  darker  and  more  con- 
spicuous than  the  others  (probably  because  it  is  not  drawn  out 
along  the  spindle  fibers)  and  owing  to  this  circumstance  its  history 
during  these  stages  may  be  followed  with  an  ease  and  certainty 
of  which  the  figures  give  but  an  imperfect  idea.  As  the  bipartite 
chromosomes  separate  in  the  anaphases  the  chromsome  in  ques- 
tion is  usually  left  lagging  near  the  equator  of  the  spindle,  though 
not  infrequently  it  lies  in  one  of  the  daughter  groups  (figs.  5,  h-ri) . 
In  the  late  anaphases,  as  the  cell-body  is  dividing,  it  may  be  seen 
passing,  without  constriction,  diminution  in  size,  or  other  sign 
of  division,  into  one  of  the  daughter-nuclei  (fig.  50).  I  desire 
especially  to  emphasize  the  fact  that  these  processes  are  seen 
with  such  clearness  and  in  so  great  a  number  of  cells,  as  to  leave 
not  the  remotest  'doubt  that  this  chromosome  neither  divides 
nor  separates  from  an  accompanying  mate.  It  is  therefore  a 
typical  odd  or  accessory  chromosome,  or  unpaired  idiochromo- 
some,  identical  in  its  general  history  with  that  seen  in  Anasa, 
Protenor,  and  so  many  other  forms.  In  the  second  maturation- 
division,  accordingly,  one  of  the  daughter-cells  in  each  case  re- 
ceives one  chromosome  less  than  the  other;  and  since  there  are 
two  classes  of  secondary  spermatocytes,  there  are  four  classes  of 
spermatids  and  of  spermatozoa.  All  receive  nine  ordinary 
chromosomes  and  one  m-chromosome.  Two-fourths  contain  and 
two-fourths  lack  a  second  small  chromosome ;  and  each  of  these  two- 
fourths  falls  into  two  classes,  one  containing  and  one  lacking  the 
accessory  chromosome.  These  four  classes  are  readily  distinguish- 
able in  polar  views  of  rather  late  anaphases,  particularly  in  cases 
where  the  accessory  chromosome  lies  at  the  same  level  as  the  chro- 
mosomes of  one  daughter  group.  Fig.  5p,q  show  two  such  daugh- 
ter groups,  from  the  same  spindle  and  in  the  same  section.  In 
each  case  two  small  chromosomes  are  present,  and  one  group 
contains  11  chromosomes,  the  other  12.  I  could  not  find  a  single 
case  of  the  other  type  (with  10  and  11  chromosomes)  in  which 
both  daughter-groups  appear  in  the  same  spindle:  but  two  ana- 
phase  groups  from  different  spindles  are  shown  in  fig.  5r,  s,  the 
former  containing  11  chromosomes,  the  latter  10. 


STUDIES   ON   CHROMOSOMES  65 

d.    The  Growth-period  and  Maturation-prophases 

The  foregoing  facts  demonstrate  in  the  clearest  manner  that 
this  individual  of  M.  femoratus  differs  from  all  other  individuals 
of  the  genus  heretofore  examined,  with  the  exception  of  Mont- 
gomery's material  of  M.  terminalis,  in  having  an  odd  or  unpaired 
idiochromosome  (accessory  chromosome)  which  corresponds  to 
the  larger  member  of  the  pair  of  unequal  idiochromosomes  found 
in  other  individuals.  They  show  also  that  the  third  small  chro- 
mosome is  not  a  small  supernumerary  of  the  -type  found  in  other 
individuals,  and  is  nothing  other  than  a  third  m-chromosome. 
This  is  fully  borne  out  by  the  growth-period  and  prophases.  As  I 
have  indicated  in  earlier  papers,  the  m-chromosomes  are  in  general 
characterized  during  the  growth-period  by  the  fact  that  they  re- 
main univalent  (there  are  some  exceptions  to  this)  and  in  most 
cases  (of  which  Metapodius  is  one)  are  in  a  diffuse  and  light- 
staining  condition.  Further,  as  was  first  shown  by  Gross  ('04)  in 
Syromastes,  as  a  rule  they  only  conjugate  to  form  a  bivalent  in 
the  final  prophases  of  the  first  division — very  often  not  until  the 
spindle  is  formed  and  the  chromosomes  are  entering  the  equator- 
ial plate.  Such  a  late  prophase,  from  a  22-chromosome  individual 
of  the  same  species  (No.  29)  is  shown  for  the  sake  of  comparison 
in  fig.  2c,  the  two  separate  m-chromosomes  appearing  above 
and  towards  the  left.  Their  final  conjugation  always  takes  place 
at  the  center  of  the  group  (fig.  2j,  3,  e-g). 

In  individual  No.  64,  prophases  of  every  stage  are  shown  in 
hundreds  of  nuclei.  In  the  latest  stages,  after  the  nuclear  wall 
has  broken  down,  three  separate  small  chromosomes  are  shown 
(fig.  2a)  which  may  be  seen  coming  together  in  the  final  prophases 
to  form  the  small  central  triad.  Figs.  1m,  n  show  two  earlier 
stages  from  the  same  cyst  with  the  last,  one  of  them  showing  the 
beginning  of  the  spindle-formation,  the  other  an  earlier  stage 
when  the  asters  are  very  small  and  often  invisible.  Each  of  these 
shows  the  three  separate  small  chromosomes,  as  before.  At  this 
time  all  the  chromosomes  are  compact  and  deeply  stained.  In 
still  earlier  stages,  at  a  time  when  the  bivalents  are  all  diffuse  and 
appear  in  the  form  of  lightly  staining  double  crosses,  rods,  etc., 

THE  JOURNAL  OP   EXPERIMENTAL.  ZOOLOGY,   VOL.  9,    NO.   1. 


66  EDMUND   B.    WILSON 

the  three  small  chromosomes  are  still  easily  seen  in  many  of  the 
nuclei;  but  they  are  now  pale  and  diffuse  like  the  bivalents.  In 
this  respect  the  third  small  chromosome  differs  from  a  "  super- 
numerary" of  the  type  described  in  my  former  paper,  and  agrees 
exactly  with  the  m-chromosomes. 

Each  nucleus  contains  at  this  period  a  single  compact,  rounded 
and*  intensely  staining  chromatic  nucleolus,  which  is  no  doubt 
the  odd  or  accessory  chromosome  (monosome),  as  in  so  many 
other  forms,4  and  in  addition  there  is  present  a  conspicuous,  rounded 


4  This  identification  is  in  agreement  with  that  of  most  observers  in  recent  years. 
A  few  writers  have  however  disputed  the  view  that  the  chromatic  nucleolus  of 
the  growth  period  of  the  spermatocytes  is  a  chromosome — e.  g.,  Moore  and 
Robinson  in  case  of  the  cockroach  ('05),  Foot  and  Strobell  in  the  case  of  Anasa('07) 
and  Euschistus  ('09),  and  Arnold  ('08),  in  case  of  Hydrophilus.  The  results  of 
Moore  and  Robinson  on  this  point  are  opposed  by  those  of  Stevens  ('05),Wassilieff 
('07),  and  more  particularly  by  the  detailed  observations  of  Morse  ('09).  Those 
of  Foot  and  Strobell  on  Anasa  are  not  sustained  by  the  later  ones  of  Lefevre  and 
McGill  ('08).  Among  others  who  have  in  the  past  two  years  adhered  to  the  view 
here  adopted  may  be  mentioned  Otte  ('07),  Davis  ('08),  Boring  ('07),  Jordan  ('08), 
Stevens  ('08,  '09),  McClung  ('08),  Robertson  ('08),  Randolph  ('08),  Nowlin  ('08), 
Payne  ('09,)  Wilson  ('096,  '09c),  Gutherz  ('09),  Wallace  ('09),  Gerard  ('09),  and 
Buchner  ('09a).  Since  I  intend  to  return  to  the  subject  hereafter  I  will  take  this 
occasion  for  only  brief  comment  on  some  of  these  results,  without  attempting  a 
full  review  of  the  literature. 

Moore  and  Robinson,  who  have  been  followed  by  Arnold  (Strasburger,  '07,  '09. 
expresses  the  same  opinion)  also  regard  the  body  that  is  seen  passing  to  one  pole  in 
one  of  the  maturation  divisions  ("accessory  chromosome")  as  not  a  chromosome 
but  a  '  'nucleolus."  I  find  it  incredible  that  anyone  can  hold  to  such  a  view  who 
reckons  squarely  with  the  large  existing  body  of  direct  and  detailed  observation 
upon  the  accessory  chromosome  itself;  and  this  view  seems  to  be  quite  ruled  out  of 
court  by  comparative  studies  on  the  sex-chromosomes,  such  for  instance  as  those 
of  Payne  on  Gelastocoris  and  the  reduvioids.  I  will  not  enter  here  upon  the  maze 
of  difficulties  regarding  the  numerical  relations  of  the  chromosomes  which  the 
same  view  involves,  since  they  have  already  been  indicated  by  Gutherz  ('09),  in  a 
recent  reply  to  Strasburger.  My  own  preparations,  including  an  extensive  series 
of  sections  and  smears  especially  of  Protenor,  Lygaeus  and  Pyrrhocoris  leave  in 
my  mind  not  the  least  doubt  of  the  identity  of  the  chromatin-(chromosome)- 
nucleolus  of  the  growth  period  with  the  odd  chromosome  (monosome)  of  the 
spermatogonia,  and  with  the  heterotropic  or  accessory  chromosome  of  the  mat- 
uration-divisions. 

Certain  writers  have  seemed  to  take  it  for  granted  that  the  accessory  chromosome 
or  "monosome"  is  always  characterized  by  its  nucleolus-like  condition  in  the  rest- 
ing nuclei,  not  only  in  the  spermatocytes  but  also  in  the  spermatogonial  and  other 


STUDIES   ON    CHROMOSOMES  67 

pale  plasmosome  which  is  considerably  larger  than  the  chromo- 
some nucleolus.  This  body,  particularly  well  shown  in  these 
slides,  is  at  once  recognizable  by  its  smooth  contour,  spheroidal 
form  (sometimes  double,  as  in  fig.  II)  and  pale  yellowish  color 
after  the  hsematoxylin,  and  it  forms  a  striking  contrast  to  the 
intense  blue-black  of  the  chromosome-nucleolus.  Nuclei  in 
which  all  five  bodies — the  three  small  chromosomes  and  both 


diploid  nuclei.  This  assumption — which  is  doing  much  to  confuse  the  whole  sub- 
ject— may  accord  with  the  facts  in  certain  species,  but  certainly  is  not  generally 
true.  Much  of  the  recent  work  in  this  field,  as  well  as  some  of  the  earlier  (e.  g.,  that 
of  McClung  '00,  and  Button  '00)  goes  to  show  that  in  many  species  it  is  only  in 
the  growth-period  of  the  spermatocytes  that  this  chromosome  forms  a  chromosome 
nucleolus,  not  in  the  diploid  nuclei  of  either  sex.  Such  seems  to  be  the  case  in  all 
the  Hemiptera  that  I  have  studied.  In  these  animals  the  accessory  chromosome, 
or  its  homologue  the  large  idiochromosome,  first  assumes  the  nucleolus-like  con- 
dition in  the  post-spermatogonial  stages,  when  its  origin  from  an  elongate  chromo- 
some may  in  some  species  readily  be  followed  step  by  step,  as  I  have  shown  in 
Lygaeus  ('056)  and  Pyrrhocoris  ('096).  In  the  spermatogonia  of  these  animals 
this  chromosome  does  not  differ  visibly  in  behavior  from  the  others  and  cannot 
be  seen  in  the  resting  nuclei. 

Several  years  ago,  in  two  successive  papers  ('05a,  '06)  I  described  and  commented 
on  the  interesting  fact  that  in  the  female  this  chromosome  (and  its  fellow,  when 
present)  seems  in  some  species  not  to  assume  a  nucleolus-like  condition  in  the 
synaptic  stage  and  early  growth-period  of  the  oocytes.  Since  some  doubts  on 
this  point  were  raised  in  my  own  mind  by  the  later  work  of  Stevens  ('06)  and  Gut- 
herz('07)I  am  now  glad  to  have  the  very  positive  confirmation  of  my  results  given 
by  the  work  of  Foot  and  Strobell  ('09)  on  Euschistus  (one  of  several  forms  I  had 
examined).  This  confirmation  must  have  been  made  without  knowledge  of  my 
previous  work,  since  the  latter  is  referred  to  in  neither  text  nor  literature  list,  and 
the  supposedly  new  facts  are  made  the  main  basis  for  renewed  attack  upon  my 
general  conclusions.  On  the  other  hand  Buchner  ('09a)  has  recently  found  in  the 
synaptic  or  "bouquet"  stage  of  the  oocytes  in  Gryllus  a  nucleolus-like  "accessory 
body"  which  he  believes  to  be  of  the  same  nature  as  the  accessory  chromosome 
of  the  male,  though  its  history  in  maturation  was  not  followed  out,  nor  is  other 
proof  of  the  conclusion  given. 

It  is  of  some  psychological  interest  to  find  Buchner  on  the  one  hand  and  Foot 
and  Strobell  on  the  other  disputing  my  conclusions  regarding  sex-production  on 
diametrically  opposing  grounds,  the  first-named  author  because  (as  he  believes) 
a  chromosome-nucleolus  is  present  in  the  oocyte-nucleus,  the  last  named  because  it 
is  absent(\).  In  what  way  either  of  the  mutally  contradictory  arguments  inval- 
idates or  weakens  my  conclusions  I  am  not  yet  able  to  perceive,  nor  need  we  here 
consider  the  contradiction  in  the  data;  but  it  is  interesting  to  observe  how  each 
of  the  arguments  goes  awry  by  reason  of  the  confusion  regarding  the  chromosome- 
nucleolus,  referred  to  above.  Foot  and  Strobell,  for  example,  argue  that  because 


68  EDMUND    B.    WILSON 

nucleoli — are  visible  in  the  same  section  are  not  very  common. 
Two  such  cases  are  shown  in  figs.  Ik,  L,  each  of  which  shows  also 
four  of  the  nine  larger  bivalents. 

I  have  not  endeavored  to  make  an  exhaustive  study  of  the 
growth-period  as  a  whole,  but  the  facts  reported  above  taken  in 

such  a  body  is  not  present  in  the  oocyte-nucleus,  therefore  the  odd  or  accessor}' 
chromosome  of  the  male  cannot  be  derived  in  fertilization  from  the  egg-nucleus 
— an  obvious  non  sequitur.  Buchner's  argument,  based  upon  precisely  opposite 
data,  shows  a  somewhat  similar,  though  less  obvious  entanglement.  The  essence 
of  his  objection  is  given  in  the  following  passage,  which  at  the  outset  accepts 
all  the  essential  facts  on  which  the  conclusions  of  Stevens  and  myself  were  based. 
"Auf  alle  Falle  haben  wir  nur  eine  Sorte  von  Eiern,  denn  dasser  (the  accessory 
chromosome)  in  einem  Ei  ausgestossen  und  im  andern  innenbehalten  wird,  er- 
scheint  undenkbar.  Die  Spermatozoen  haben  das  accessorische  Chromosom  zur 
Halfte.  Nehmen  wir  an,  die  Eier  besassen  das  accessorische  Chromosom  schon, 
so  gabe  es  Tiere  mit  zwei  Monosomen  und  solche  mit  einem — ein  Fall  der  nicht 
ezistiert"  (op.  cit.,  p.  409,  italics  mine).  This  is,  indeed,  an  astounding  statement; 
for  it  was  the  very  fact  that  there  are  individuals  that  have  but  one  monosome 
or  accessory  chromosome  (the  males),  and  other  individuals  of  the  same 
species  (the  females)  that  have  two  corresponding  chromosomes,  upon  which  the 
conclusions  of  Stevens  and  myself  were  mainly  based  (!).  This  is  true,  asGutherz 
('08)  has  shown,  of  the  very  form  (Gryllus)  of  which  Buchner  is  writing,  the  single 
odd  chromosome  (monosome)  of  the  male,  recognizable  by  its  peculiar  form  and 
other  characters,  being  represented  in  the  female  by  two  such  chromosomes. 
This  is  also  in  agreement  with  the  results  of  other  recent  workers  on  the  Orthoptera 
including  Wassilieff,  Davis,  Jordan  and  Morse.  I  can  therefore  find  no  meaning 
in  Buchner's  statement  unless  the  word  "Monosom"  be  used  to  denote  simply 
a  chromosome-nucleolus,  when  the  passage  becomes  at  least  intelligible.  But 
such  a  restriction  in  the  meaning  of  this  word  is  not  justified  by  its  etymology, 
by  the  original  definition  of  its  author  (Montgomery,  '06a,  '066)  nor  by  the  facts; 
and  it  does  not  seem  to  accord  even  with  Buchner's  own  usage  elsewhere  in  the 
paper.  That  Buchner's  statement  is  totally  at  variance  with  the  facts  when  cor- 
rectly stated  is  shown  by  the  following  summary  of  my  results,  quoted  from  one 
of  the  papers  in  which  Montgomery  first  defined  the  word  "monosome."  "When 
there  is  a  single  monosome  in  the  spermatogenesis  (as  in  Protenor,  Harmostes, 
Anasa  and  Alydus)  there  are  two  in  the  ovogenesis  so  that  the  ovogonia  possess 
always  an  even  number  of  chromosomes"  ('066,  p.  145,  italics  mine). 

But  even  if  we  admit  that  the  "accessory  body"  of  the  female  is  a  chromosome — 
and  not  only  is  there  no  proof  of  this  but  many  reasons  for  doubting  it — what  ad- 
verse bearing  would  the  fact  have  upon  the  "theory"?  None  as  far  as  I  can  see, 
unless  this  chromosome  were  proved  to  be  univalent  and  without  a  synaptic  mate 
Were  all  this  true,new  and  unintelligible  complications  would  arise  in  regard  to  the 
numerical  relations  of  the  diploid  and  haploid  chromosome-groups  in  both  sexes; 
but  it  is  not  worth  while  to  consider  these  puzzles  since  they  lie  in  a  region  not  of 
observed  fact  but  of  pure  phantasy. 


STUDIES   ON    CHROMOSOMES  69 

connection  with  the  spermatocyte-divisions,  are  thoroughly  deci- 
sive in  showing  the  third  small  chromosome  to  be  an  extra  ra-chro- 
mosome  not  distinguishable  in  any  respect  from  the  other  two. 

2    DISCUSSION 

It  seems  to  me  that  in  the  individual  of  Metapodius  that  has 
here  been  described  nature  has  performed  an  experiment  which, 
as  far  as  it  goes,  is  precisely  such  as  we  should  like  to  carry  out 
artificially  in  order  to  test  the  hypothesis  of  the  genetic  continuity 
of  the  chromosomes  and  the  question  of  their  qualitative  relations 
in  the  maturation-process.  .  The  experiment  (if  we  may  call  it 
such)  consisted  in  the  omission  from  the  typical  22-chromosome 
diploid  groups  of  the  small  idiochromosome,  and  its  replacement 
by  one  of  different  type,  a  third  m-chromosome.  In  what  way 
this  was  effected  can  only  be  conjectured;  but  it  seems  altogether 
probable  that  the  anomaly  was  present  in  the  original  fertilized 
egg,  as  a  result  of  one  preexisting  in  one  or  both  the  gamete- 
nuclei.5  In  any  case  we  may  be  sure  that  it  arose  very  early  in  the 
ontogeny,  at  a  period  prior  to  the  separation  of  the  right  and  left 
gonads,  since  both  testes  show  precisely  the  same  characters. 

It  is  certain  that  the  initial  anomaly  has  persisted  unchanged 
through  many  generations  of  cells,  and  that  the  alteration  in  the 
diploid  groups  has  involved  corresponding  modifications  in  the 
maturation-process.  The  significant  fact  is  that  throughout  this 
process  the  chromosome  that  has  been  added  does  not  take  the  place  of 
the  one  that  has  been  omitted,  but  behaves  according  to  its  own  kind. 
This  is  a  truly  remarkable  result  when  we  consider  that  the  num- 
ber of  chromosomes  in  the  diploid  groups  (22)  remains  unaltered. 
These  groups  still  consist  of  11  pairs  of  chromosomes;  but  one  is 

5  We  must  assume,  in  this  case,  that  the  sperm-nucleus  contained  no  small  idio- 
chromosome and  that  either  this  nucleus  or  the  egg-nucleus  contained  two  m-chro- 
mosomes.  The  former  condition  may  have  resulted  from  a  failure  of  the  idiochro- 
mosomes  to  separate  in  the  second  spermatocyte-division  (which,  as  I  have  shown, 
may  actually  occur).  The  presence  of  a  second  w-chromosome  may  be  due  to  a 
similar  cause. 


70  EDMUND   B.    WILSON 

an  unnatural  or  hybrid  pair,  which  consists  of  non-homologous 
members — the  large  idiochromosome  and  the  third  m-chromosome. 
The  facts  show  most  decisively  that  these  two  chromosomes  do 
not  play  the  part  of  synaptic  mates  towards  each  other,  but  retain 
each  its  own  characteristic  behavior.  In  synapsis  the  third 
m-chromosome  invariably  couples  with  its  own  kind  to  form  a 
triad  element  while  the  large  idiochromosome  remains  unpaired. 
Thus  the  substitution  of  one  chromosome  for  another  of  a  different 
kind  has  been  followed  by  no  regulative  process,  and  a  perma- 
nently new  combination  has  been  produced.  The  full  force  of  this 
conclusion  first  becomes  evident  when  we  compare  the  present  case 
with  those  in  which  there  is  present  a  single  small  supernumerary 
of  the  type  described  in  my  fifth  Study.  In  the  diploid  groups  such 
a  supernumerary  is  quite  indistinguishable  from  a  third  m-chro- 
mosome— as  we  may  see,  for  instance,  upon  comparison  with  figs. 
Ik,  7,  t-y,  photograph  29,  of  Study  V.6  In  the  first  spermatocyte- 
division  also,  in  cases  where  a  small  supernumerary  lies  within  the 
ring  of  large  bivalents  (as in  photograph  6,  fig.  7i  of  my  fifth  Study) 
side-views  give  a  picture  almost  indistinguishable  from  such  a 
condition  as  that  shown  in  fig.  3c.  Such  a  side  view  of  terminalis 
No.  1,  is  given  for  comparison  in  fig.  3d,  the  two  m-chromosomes 
just  separating  at  the  center  of  the  group,  and  the  supernumerary 
(s)  just  to  the  left.  The  resemblance  between  this  figure  and  fig. 
3c  is  so  close  as  to  amount  almost  to  identity.  It  seems  incredible 
that  the  behavior  of  the  third  small  chromosome  in  the  ensuing 
division  should  not  be  identical  in  the  two  cases ;  and  it  should  be 
identical  were  the  history  of  the  individual  chromosomes  in  matur- 
ation determined  merely  by  their  size  or  their  mechanical  rela- 
tion to  the  achromatic  figure.  In  point  of  fact,  however,  the  small 
supernumerary  and  the  third  m-chromosome  show  characteristic 
differences  throughout  the  whole  process.  In  the  growth-period 
the  former  appears  as  a  condensed  chromosome-nucleolus,  usually 
coupled  with  the  idiochromosome-nucleolus,  while  the  m-chro- 
mosome remains  diffuse  and  usually  free.  In  the  first  division 
the  former  divides  as  a  univalent  (i.  e.,  it  is  typically  uncoupled, 
though  it  may  be  in  contact  with  the  idiochromosomes)  and  is 

8Loc.  cit.:'09c. 


STUDIES   ON   CHROMOSOMES  71 

usually  outside  the  ring  (as  in  fig.  2h) ;  while  the  third  ra-chromosome 
is  always  coupled  with  the  two  others  at  the  center  of  the  ring,  and 
moves  to  one  pole  without  division.  In  the  second  division  these 
relations  are  almost  exactly  reversed,  the  ra-chromosome  dividing 
equationally  as  a  univalent,  while  the  supernumerary  does  not 
divide  and  is  typically  coupled  with  the  idiochromosome  bivalent 
near  the  center  of  the  group.  I  desire  to  emphasize  the  fact  that 
these  differences  are  in  no  way  obscure  or  difficult  to  see,  but  are 
conspicuously  shown  in  so  great  a  number  of  cells  as  to  remove 
all  doubt.7 


7  This  point  demands  emphasis  because  of  the  scepticism  expressed  by  certain 
writers  in  regard  to  the  constancy  of  the  chromosomes  in  respect  to  number,  size 
and  behavior.  Conspicuous  among  these  writers  is  Delia  Valle  ('09)  who  has 
brought  together  a  valuable  if  somewhat  uncritical  review  of  the  literature,  and 
contributes  careful  observations  of  his  own  upon  variations  in  the  chromosome- 
number  in  the  somatic  cells  of  Salamandra.  Such  scepticism  is  perhaps  not  sur- 
prising in  view  of  the  unlucky  contradictions  that  still  exist  in  the  literature  even 
of  so  favorable  and  well  known  a  group  as  the  insects.  But  to  ascribe  this  con- 
fusion of  the  literature  to  a  confusion  of  the  facts — i.  e.,  to  an  inconstancy  so  great 
as  to  preclude  the  possibility  of  attaining  exact  results — would  be,  I  think,  a  fatal 
blunder.  The  confusion  in  the  literature  cannot,  of  course,  be  attributed  altogether 
to  mistakes  of  observation  or  to  accidents  of  technique — though  both  these  must 
be  held  to  a  strict  reckoning.  I  am  not  aware  that  anyone  has  maintained  that 
the  relations  of  the  chromosomes  form  an  exception  to  all  other  biological  phenom- 
ena in  being  absolutely  fixed  and  immutable;  and  due  weight  should  be  given 
to  the  numerical  variations  that  have  been  recorded  by  Delia  Valle  and  many  others 
myself  included.  The  fact  remains  that  it  is  possible  to  determine  accurately 
what  are  the  normal  or  typical  relations  of  the  chromosomes,  as  of  other  struc- 
tures, and  to  establish  in  many  cases  their  high  degree  of  constancy.  The  same 
common  sense  must  be  used  in  the  treatment  of  these  relations  as  in  the  case  of 
other  phenomena  that  are  subject  to  variation.  For  example,  insects  have  been 
seen  with  seven  legs,  but  it  is  not  for  this  reason  to  be  doubted  that  insects  have 
six  legs.  In  like  manner,  in  the  ovaries  of  Largus  cinctus  I  have  seen  as  many  as 
three  dividing  cells  that  show  13  chromosomes;  but  I  nevertheless  do  not  doubt, 
after  the  study  of  a  large  number  of  cases,  that  the  typical  number  is  12. 

The  case  of  Metapodius  is  disposed  of  by  Delia  Valle  in  the  following  easy  fash- 
ion. "Not  constancy  but  variability  in  the  number  of  chromosomes  is  the  general 
rule  in  all  organisms;  of  which  the  observations  published  by  him  (Wilson)  are 
but  a  special  confirmatory  case"  (op.  cit.,  p.  161,  translation).  Better  acquaintance 
with  the  facts  in  Metapodius  would  probably  render  Prof.  Delia  Valle  less  certain 
of  this;  for  I  am  confident  that  no  observer  of  ordinary  competence  could  confuse 
such  a  series  of  relations  as  that  here  displayed  with  the  occasional  fluctuations 
with  which  we  are  familiar  in  many  forms,  including  this  very  genus. 


72  EDMUND    B.   WILSON 

It  seems  to  me  that  such  facts  have  the  value  of  actual  experi- 
mental evidence  in  support  of  the  hypothesis  of  the  genetic  con- 
tinuity of  the  chromosomes  and  that  of  their  qualitative  difference. 
All  will  admit  that  the  peculiarities  of  the  later  generations  of  cells 
in  this  individual  of  Metapodius  are  inherited  from  earlier  ones. 
It  is  the  obvious,  natural  and,  I  think,  inevitable  conclusion  that 
the  third  ra-chromosome,  introduced  at  an  early  period,  has  not 
lost  its  identity  in  the  later  stages.  If  its  presence  is  merely  owing 
to  a  corresponding  excess  of  chromatin,  how  shall  we  account  for 
the  characteristic  peculiarities  of  behavior  that  differentiate  it 
so  sharply  from  an  ordinary  "  supernumerary "  of  corresponding 
size?  To  reply  that  the  excess  represents  a  particular  kind  of 
chromatin  that  is  re-segregated  at  each  division  in  the  form  of  a 
particular  chromosome  is  to  grant  the  most  vital  assumption 
in  the  hypothesis  of  genetic  continuity. 

I  think  that  sufficient  emphasis  has  not  yet  been  laid  upon  the 
support  given  to  this  hypothesis  by  the  variable  position  of  the 
chromosomes  in  the  diploid  groups.  I  have  several  times  pointed 
out  in  this  paper  and  preceding  ones,  that  there  is  no  constancy  in 
the  relative  position  of  the  spermatogonial  chromosomes — as  may 
be  seen  with  particular  clearness  in  case  of  the  ra-chromosomes  of 
the  Coreidse  or  the  small  idiochromosome  of  the  Pentatomidae  or 
Lygseidse,  or  of  chromosomes  distinguishable  by  their  large  size, 
such  as  are  seen  in  Protenor,  Largus  or  Anasa.  This  is  certainly 
not  what  we  should  expect  were  the  chromosomes  merely  "  tactic" 
formations  that  appear  in  characteristic  array,  as  a  crystal  form 
in  a  solution,  merely  because  of  the  specific  properties  of  a  single 
chromatin-substance  as  such.  Two  answers  might  be  made  to 
this.  It  might  be  said  that  the  chromosomes  merely  represent 
the  segregation  of  so  many  different  kinds  of  chromatin  that  are 
mixed  together  in  the  resting  nucleus.8  I  am  disposed  to  regard 
this  as  a  tenable  hypothesis;  but  obviously  it  grants  the  most  essen- 
tial part  of  the  continuity  hypothesis.  Again  it  might  be  said  that 
the  chromosomes  are  originally  formed  always  in  the  same  j  osi- 
tion  but  lose  it  by  subsequent  shiftings  in  the  prophases.  It 

•  Cf.  Fick:  '05;  Wilson:  '09c. 


STUDIES   ON   CHROMOSOMES  73 

would  be  difficult  to  disprove  this  in  ordinary  cases;  but  fortun- 
ately Boveri's  studies  on  Ascaris  ('09)  have  shown  beyond  all  doubt 
that  in  this  form  there  is  no  constancy  in  the  original  position  of 
the  prophase  chromosomes,  the  only  definite  order  being  shown  in 
the  close  agreement  of  each  pair  of  daughter-cells.  The  position 
of  the  prophase  chromosomes  as  Boveri  shows  with  great  cogency 
is  here  a  consequence  of  the  position  in  which  they  entered  the  nu- 
cleus in  the  preceding  telophases ;  as  the  latter  position  is  itself  due 
to  causes  (which  may  be  quite  fortuitous)  that  determine  the  posi- 
tion of  the  preceding  metaphase  chromosomes. 

The  facts  support  no  less  directly  and  strongly  the  conclusion 
that  the  chromosomes  differ  among  themselves  in  a  definite  way 
in  respect  to  their  behavior,  and  hence  in  respect  to  their  functional 
significance.  The  differences  seen  in  the  maturation-process  have 
thus  far  taught  us  nothing  whatever  in  regard  to  the  individual 
physiological  meaning  of  the  chromosomes,  in  heredity  or  other- 
wise, and  they  are  not  to  be  compared  in  value  with  the  results  of 
direct  experiment,  such  as  those  carried  out  by  Boveri  ('07)  in 
dispermic  sea-urchin  eggs.  It  is  nevertheless  of  great  interest  that 
the  results  from  these  different  sources  should  be  in  harmony.  In 
my  preceding  paper  I  have  called  attention  especially  to  the  sig- 
nificance of  the  couplings  of  the  chromosomes,  pointing  out  that 
these  certainly  do  not  depend  upon  the  size  of  the  chromosomes 
(though  those  which  couple  in  synapsis  are  in  fact  equal  members 
of  a  pair  save  in  certain  special  cases)  nor  can  they,  apparently, 
depend  upon  the  achromatic  mechanism.  The  various  combina- 
tions in  Metapodius  seem  to  arise  simply  by  the  addition  or  sub- 
traction of  certain  chromosomes  without  alteration  of  the  achro- 
matic elements;  yet  in  the  resulting  new  combinations  the  chro- 
mosomes still  behave  each  according  to  its  kind,  and  (as  previously 
indicated)  irrespective  of  their  size.  We  seem  thus  driven  to 
accept  the  view  that  the  chromosomes  are  physico-chemically  dif- 
ferent, with  all  the  consequences  which  such  a  view  may  involve. 

The  cogency  of  the  evidence  in  favor  of  the  qualitative  differ- 
ences of  the  chromosomes  brought  forward  in  Boveri's  masterly 
work  must  be  generally  recognized,  as  has  recently  been  admitted 
even  by  Driesch  ('09)  who  formerly  endeavored  to  find  a  different 


74  EDMUND    B.   WILSON 

explanation  of  the  facts.  It  will  be  evident  to  readers  of  my  for- 
mer papers  that  I  am  fully  prepared  to  accept  Boveri's  conclusions; 
but  there  is  one  very  important  fact,  finally  established  by  the 
present  paper,  that  must  be  clearly  recognized.  If  we  assume  that 
different  factors  of  heredity  are  in  some  sense  unequally  distri- 
buted among  the  chromosomes,  we  need  feel  no  surprise  that  the 
duplication  of  one  or  more  of  the  ordinary  chromosomes  should 
produce  no  perceptible  qualitative  effect  upon  development.  But 
it  is  very  surprising  that  no  visible  effect  should  be  produced  by  the 
removal  of  a  particular  chromosome  that  has  no  duplicate  to  take 
its  place.  In  preceding  papers  I  have  called  attention  to  the  sing- 
ular fact  that  Montgomery's  material  of  M.  terminalis  differs  con- 
sistently from  my  own  in  the  lack  of  the  small  idiochromosome  or 
uY-element"  (see  Wilson  '09a,  for  this  term);  but  the  possibility 
of  two  distinct  species  or  races  having  been  confused  could  not  be 
absolutely  excluded.  In  the  present  case,  however,  no  doubt  can 
exist,  since  the  two  original  specimens  of  M.  femoratus  from  Miami, 
Florida,  are  in  my  cabinet,  and  in  perfect  condition  for  identifica- 
tion. One  of  these,  as  already  stated,  contains  both  the  small 
idiochromosome  and  an  additional  supernumerary,  while  both  are 
lacking  in  the  other  individual;  yet  the  two  individuals  seem  to  be 
otherwise  in  every  essential  respect  identical.  All  doubt  is  thus 
removed  that  the  small  idiochromosome  or  Y-element,  which  forms 
the  synaptic  mate  of  the  accessory  chromosome  or  X-element, 
may  be  present  in  some  individuals  and  absent  in  others  of  the 
same  species,  without  the  appearance  of  any  corresponding  dif- 
ferences in  the  sexual  or  other  characters  as  far  as  shown  in  the 
external  morphology  of  the  animal.9  This  chromosome,  as  shown 

9  It  will  be  seen  from  this  how  readily  discrepancies  regarding  the  number  of 
chromosomes  might  arise  between  different  observers  working  on  the  same  species. 
It  might  seem  that  we  have  here  a  simple  and  plausible  explanation  of  the  contra- 
dictions that  have  arisen  in  the  case  of  Anasa  tristis;  for  we  might  assume  that  the 
diploid  number  is  21  in  some  individuals  of  this  species  and  in  others  22 ;  and  a  simi- 
lar explanation  has  in  fact  already  been  adopted  by  more  than  one  recent  writer 
(cf.  Delia  Valle:  '09,  Buchner:  '096). 

I  am  not  myself  able  to  take  this  view  of  this  particular  case  for  several  reasons. 
In  the  first  place,  if  there  be  individuals  of  this  species  that  have  22  spermatogonial 
chromosomes,  as  maintained  by  Foot  and  Strobell  ('07)  we  should  expect  to  see 


STUDIES   ON   CHROMOSOMES  75 

by  the  work  of  Stevens  and  myself,  is  widely  distributed  among  the 
insects  (Hemiptera,  Coleoptera,  Diptera)  and  is  strictly  confined 
to  the  male  line  (except  when  supernumeraries  are  present).  In 
species  having  an  odd  or  accessory  chromosome  the  Y-element 
(small  idiochromosome)  is  wanting,  and  I  have  urged  this  fact  as 
showing  that  this  latter  chromosome  cannot  play  any  essential  role 
in  sex-production10  or  in  the  transmission  of  the  secondary  sexual 
characters,  as  Castle  ('09)  ingeniously  suggests.  What  I  desire 
here  to  point  out  is  that  by  parity  of  reasoning  we  should  also  con- 
clude that  this  chromosome  is  devoid  of  any  special  significance  in 
heredity  of  any  kind,  at  least  as  far  as  the  visible  external  charac- 

12  instead  of  11  separate  chromosomes  in  the  first  spermatocyte  division,  as  we  do 
in  the  22-chromosome  individuals  of  Metapodius (Wilson:  '09c).  Throughout  the 
Hemiptera,  indeed,  when  the  accessory  chromosome  (or  its  homologue,  the  large 
idiochromosome)  is  accompanied  by  a  synaptic  mate  or  Y-element,  the  two  are 
separate  in  the  first  division,  which  accordingly  shows  one  more  than  the  reduced 
or  haploid  number — i.e.,  *+!•  The  photographs  of  Foot  and  Strobell  show,  how- 
ever, 11  chromosomes  in  this  division  (the  two  m-chromosomes  being  of  course 
counted  as  one,  like  the  other  bivalents),  as  they  should  if  the  spermatogonial  num- 
ber be  21. 

Still, this  might  be  a  case  like  that  of  Syromastes,  where  no  Y-element  is  present, 
but  the  accessory  is  itself  double — though  such  a  parallel  would  hardly  help  the 
case,  since  in  no  form  is  the  failure  of  the  accessory  to  divide  in  one  division  more 
indubitably  shown  than  in  Syromastes,  while  Foot  and  Strobell  are  persuaded  that 
it  does  divide  in  Anasa.  But,  secondly,  my  own  extended  additional  observation, 
like  the  studies  of  Lefevre  and  McGill  ('08),  still  continues  to  give  but  one  result 
as  before.  The  living  animals  (from  the  same  locality  as  the  material  of  Foot  and 
Strobell)  have  been  kept  by  hundreds  in  the  greenhouse  for  months  at  a  time  in 
successive  years,  and  have  been  regularly  employed  for  class  work  in  cytology  and 
for  experimental  purposes,  in  the  course  of  which  large  numbers  of  additional 
sections  and  smears  have  been  prepared  and  examined.  Others  as  well  as  myself 
have  carefully  searched  among  these  preparations  for  cases  showing  more  than  21 
spermatogonical  chromosomes,  without  success  apart  from  the  double  or  multiple 
groups  that  occasionally  appear.  The  same  relation  continually  recurs,  namely 
21  spermatogonial  chromosomes  of  which  three  are  larger  than  the  others,  while 
in  the  dividing  ovarian  cells  the  number  22  appears  with  equal  constancy.  That 
not  even  one  case  of  22  spermatogonial  chromosomes  has  thus  far  been  found  is 
indeed  surprising;  for  plus  variations  in  the  diploid  groups  are  known  to  occur  in 
some  species  of  Hemiptera,  and  I  have  myself  described  such  cases  (e.  g.,  Wilson: 
'09c). 

These  and  other  reasons  lead  me  to  believe  that  the  conclusions  of  Foot  and  Stro- 
bell were  based  on  the  observation  either  of  very  rare  fluctuations  in  the  normal 
diploid  number  or  of  accidental  products  of  the  technique. 

"  Loc.  cit.:  '06,  '09a,  '09d. 


76  EDMUND    B.  WILSON 

ters  and  the  more  obvious  internal  features  are  concerned.  Taken 
by  itself  this  case  may  seem  to  form  a  legitimate  piece  of  evidence 
against  Boveri's  theory.  It  cannot,  howevei ,  be  taken  alone,  but 
must  be  viewed  from  the  more  general  standpoint  given  by  the 
evidence  as  a  whole.  We  are  still  too  ignorant  of  the  significance 
of  the  "  sex-chromosomes  "  to  form  an  adequate  opinion  as  to  the 
meaning  of  the  Y-element.  It  may  be  in  this  case  (as  I  earlier 
suggested)  a  degenerating  element,  or  may  represent  an  excess  of 
a  chromatin  that  is  duplicated  elsewhere  in  the  chromosome- 
group;  but  these  and  other  speculative  possibilites  that  suggest 
themselves  may  well  await  the  outcome  of  further  study. 

Department  of  Zoology,  Columbia  University,  December  23,  1909. 


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genesis of  Periplaneta  Americana.     Q.  J.  M.  S.,  48,  4. 

NOWLIN,  N.     1908    The  Chromosome  Complex  of  Melanoplus  bivittatus.     Sci. 
Bull.,  Univ.  Kansas,  4, 12. 

OTTE,  H.     1907    Samenr,eifung  und  Samenbildung  bei  Locusta  viridissima.     Zool. 
Jahrb.,  Anat.  u.  Ontog.,  24. 

PAULMIER,  F.  C.     1899    The  Spermatogenesis  of  Anasa  tristis.     Journ.  Morph., 
15,  Suppl. 

PAYNE,   E.     1909    Some   New  Types   of   Chromosome   Distribution   and  Their 
Relation  to  Sex.     Biol.  Bull.  16,  4. 

PINNEY,  E.     1908    Organization  of  the  Chromosomes  in  Phrynotettix  magnus. 
Sci.  Bull.,  Univ.  Kansas,  4,  14. 

RANDOLPH,  H.     1908    On  the  Spermatogenesis  of  the  Earwig,  Anisolaba  maritima. 
Biol.  Bull.,  15,  2. 


78  EDMUND   B.    WILSON 

ROBERTSON,  W.  R.  B.     1908    The  Chromosome  Complex  of  Syrbula  admirabilis. 
Sci.  Bull.,  Univ.  Kansas,  4,  13. 

STEVENS,  N.  M.     1905    Studies  in  Spermatogenesis  with  Especial  Reference  to 
the  "Accessory  Chromosome."     Carnegie  Inst.,  Wash.,  Pub.  no.  36. 

1906  A  Comparative  Study  of  the  Heterochromosomes  in  Certain 
Species  of  Coleoptera,  Hemiptera  and  Lepidoptera,  with  Especial 
Reference  to  Sex  Determination.  Ibid.,  No.  36,  Part  2. 

1908  A  Study  of  the  Germ-cells  of  Certain  Diptera,  with  Reference 
to  the  Heterochromosomes  and  the  Phenomena  of  Synapsis.     Journ. 
Exp.  Zool.,  5,  3. 

1909  Further  Studies  on  the  Chromosomes  of  Coleoptera.     Ibid.,  6, 1. 

STRASBURGER,  E.     1907    Ueber  die  Individuality  der  Chromosomen  und  die 
Pfropfhybridenfrage.     Jahrb.  Wiss.  Bot.,  44,  3. 

1909  Histologische  Beitrage,  7,  Zeiptunkt  der  Bestimmung  des 
Geschlechts,  Apogamie,  Parthenogenesis  und  Reduktionsteilung. 
Jena,  1909. 

SUTTON,  W.  S.     1900     The   Spermatogonial   Divisions  in   Brachystola  magna. 
Kansas  Univ.  Bull.,  1,  3. 

WALLACE,  L.  B.     1909    The  Spermatogenesis  of  Aglaena  nsevia.     Biol.  Bull.,  17,  2- 

WASSILIEFF,  A.     1907    Die  Spermatogenese  von  Blatta  germanica.    Arch.  Mik- 
Anat.,  70. 

WILSON,  E.  B.     1905a    The  Chromosomes  in  Relation  to  the  Determination  of 
Sex.     Science,  n.  s.,  27,  564. 

19056  The  Behavior  of  the  Idiochromosomes  in  Hemiptera.  Stud- 
ies on  Chromosomes,  I.  Journ.  Exp.  Zool.,  2,  3. 

1905c  The  Paired  Microchromosomes,  Idiochromosomes  and  Hetero- 
tropic  Chromosomes  in  Hemiptera.  Studies  on  Chromosomes,  II. 
Journ.  Exp.  Zool.,  2,  4. 

1906  The  Sexual  Differences  of  the  Chromosomes  in  Hemiptera,  etc. 
Studies  on  Chromosomes,  III.     Ibid.,  3,  1. 

1907  The  Case  of  Anasa  tristis.     Science,  n.  s.,  25,  631. 

1908  The  Accessory  Chromosome  of  Anasa  tristis.     Report  of  Am. 
Soc.  Zool.,  Science,  n.  s.,  27,  690. 

1909a    Recent  Researches  on  the  Determination  and  Heredity  of 

Sex.     Vice-presidential  Address,  A.  A.  A.  S.,  Science,  n.  s.,  732. 

19096    The  Accessory  Chromosome  in  Syromastes  and  Pyrrhocoris. 

Studies  on  Chromosomes,  IV.     Journ.  Exp.  Zool.,  6,  1. 

1909c    The  Chromosomes  of  Metapodius.     A  Contribution  to  the 

Hypothesis  of  the  Genetic  Continuity  of  Chromosomes.     Studies  on 

Chromosomes,  V.     Ibid.,  6,  2. 

1909d    Secondary  Chromosome-couplings  and  the  Sexual  Relations 

in  Abraxas.     Science,  n.  s.,  29,  748. 


STUDIES  ON  CHROMOSOMES 

VII.  A  Review  of  the  Chromosomes  of  Nezara; 
with  Some  More  General  Considerations 


By   EDMUND  B.  WILSON 
Zoological  Department,  Columbia  University 

Reprinted  from  THE  JOURNAL  OF  MORPHOLOGY 
Vol.  22,  No.  1,  March  20,  1911 


Reprinted  from  JOURNAL  OF  MORPHOLOGY,  VOL.  22,  No.  1 
MARCH,  1911 


STUDIES  ON  CHROMOSOMES 

VII.      A    REVIEW    OF   THE   CHROMOSOMES    OP  NEZARA;    WITH   SOME 
MORE  GENERAL  CONSIDERATIONS 

EDMUND  B.  WILSON 

From  the  Zoological  Department,  Columbia  University 

NINE  FIGURES  AND  ONE  PLATE 

CONTENTS 

Introduction 1 71 

Descriptive 73 

1  The  second  spermatocyte-division  in  Nezara 73 

a  The  idiochromosomes 73 

b  The  double  chromosome 77 

2  The  first  spermatocyte-division 78 

3  The  growth-period  and  spermatocyte-prophases 80 

4  The  diploid  chromosome-groups 83 

General 84 

5  The  idiochromosomes 84 

a  Composition  and  origin  of  the  XY-pair 85 

b  Modifications  of  the  X-element 88 

c  Sex-limited  heredity 94 

d  Secondary  sexual  characters 99 

6  Modes  in  which  the  chromosome-number  may  change 99 

Conclusion 105 

INTRODUCTION 

In  the  first  of  these  'Studies'  ('05a)  I  described  the  idiochromo- 
somes (X  and  Y-chromosomes)  of  Nezara  hilaris  as  being  of  equal 
size  in  the  male,  and  reached  the  conclusion  that  in  this  species 
no  visible  dimorphism  appears  in  the  spermatid-nuclei.  In  my 
third  'Study'  ('06),  after  examination  of  the  female  diploid  groups, 
this  species  was  assigned  a  unique  position  as  the  single  then 

71 


72  EDMUND    B.    WILSON 

known  representative  of  a  type  in  which  a  pair  of  idiochromo- 
somes  can  be  identified  in  both  sexes,  but  are  of  equal  size  in 
both,  and  in  which,  accordingly,  no  visible  sexual  differences  ap- 
pear in  the  diploid  nuclei.  These  conclusions,  as  is  now  appar- 
ent, were  based  upon  a  wrong  identification  of  the  idiochromo- 
some-pair,  which  is  not  the  smallest  pair,  as  I  then  believed,1  but 
one  of  the  largest.  When  this  fact  was  recognized,  the  true  con- 
ditions soon  became  evident. 

I  was  led  to  re-examine  Nezara  hilaris  by  the  fact  (very  sur- 
prising to  me)  that  in  Nezara  viridula,  a  southern  species  closely 
similar  to  N.  hilaris,  the  idiochromosomes  of  the  male  are  ex- 
tremely unequal  in  size,  and  the  dimorphism  of  the  spermatid- 
nuclei  is  correspondingly  marked.  Upon  returning  to  the  study 
of  N.  hilaris  it  soon  became  manifest  that  the  dimorphism  is 
present  in  this  species  also,  though  in  far  less  conspicuous  form. 
The  size-difference  between  the  X-  and  Y-chromosomes  is  here 
often  so  slight  that  I  did  not  at  first  distinguish  it  from  an  incon- 
stant fluctuation  of  size,  such  as  is  sometimes  seen  between  the 
members  of  the  other  chromosome-pairs.  When,  however,  the 
identity  of  the  XY-pair  was  correctly  recognized,  its  constancy 
of  position  and  of  size  in  the  second  division  enabled  me  to  make 
an  accurate  comparison  between  it  and  the  other  bivalents;  and 
this  fully  established  the  constant  inequality  of  its  members, 
which  is  constantly  greater  than  that  now  and  then  seen  in  other 
pairs.  Both  species  also  exhibit  some  other  very  interesting 
features  that  I  overlooked  in  my  former  studies. 

Nezara  can  therefore  no  longer  stand  as  a  representative  of 
the  third  of  the  types  distinguished  in  my  third  'Study,'  but 
belongs  with  Euschistus,  Lygaeus,  etc.,  in  the  second  type 

1  This  was  in  part  because  in  most  of  the  other  forms  known  at  the  time  the 
idiochromosomes  are  in  fact  the  smallest,  or  one  of  the  smallest,  pairs.  In  part, 
also,  I  followed  Montgomery  ('01)  who  described  in  this  species  two  small  "  chro- 
matin  nucleoli"  in  the  spermatogonial  groups,  and  believed  them  to  be  identical 
with  the  chromatic  nucleolus  of  the  growth-period.  In  a  later  paper  ('06)  Mont- 
gomery states  these  "chromatin  nucleoli"  to  be  "apparently  not  quite  equal  in 
volume,"  and  asserts  that  I  was  in  error  in  describing  them  as  equal.  In  my 
material  they  are  certainly  equal  in  the  great  majority  of  cases.  However,  this 
is  not  the  idiochromosome-pair. 


STUDIES   ON   CHROMOSOMES  73 

DESCRIPTIVE 

Since  the  two  species  agree  very  closely  save  in  respect  to  the 
idiochromosomes  they  may  conveniently  be  considered  together. 
Before  describing  the  divisions,  attention  may  be  called  to  a 
striking  difference  between  the  two  species  in  respect  to  the  size 
of  the  cells  and  karyokinetic  figures.  As  a  comparison  of  the 
figures  will  show,  the  spermatocytes  and  maturation  division- 
figures  of  N.  hilaris  are  much  larger  than  those  of  N.  viridula. 
In  the  spermatogonia  this  difference  is  also  apparent,  though  less 
marked.  In  the  ovaries,  strange  to  say,  it  cannot  certainly  be 
detected,  either  in  the  dividing  cells  or  in  the  nuclei  of  the  follicle- 
cells  or  of  the  tip-cells  at  the  upper  end  of  the  ovary.  It  would  be 
interesting  to  make  a  more  accurate  study  of  these  relations; 
but  I  will  here  only  state  that  the  differences  between  the  two 
species  seem  to  arise  mainly  through  greater  growth  of  the 
spermatocytes  in  N.  hilaris.  With  this  is  correlated  a  greater 
size  of  the  testis  as  a  whole;  but  the  size  of  the  entire  body  in  this 
species  is  but  little  larger,  as  far  as  I  have  observed,  than  in  N. 
viridula. 

As  regards  the  general  features  of  the  divisions,  the  diploid 
groups  of  both  sexes  uniformly  contain  fourteen  chromosomes, 
the  first  spermatocyte-division  eight  and  the  second  seven,  the 
idiochromosomes  being,  as  is  the  rule  in  Hemiptera,  separate  and 
univalent  in  the  first  division. 

1.     The  second  spermatocyte-division 

a.  The  idiochromosomes.  Polar  views  of  the  second  division 
always  show  7  chromosomes  which  are  usually  grouped  in  an 
irregular  ring  of  six  with  the  seventh  near  its  center  (fig.  3  j-m, 
figs.  14,  15).  In  both  species  one  chromosome  of  the  outer  ring 
(s)  can  usually  be  distinguished  as  the  smallest,  though  this  is 
not  always  evident  owing  to  the  apparent  variations  produced 
by  different  degrees  of  elongation.  This  is  the  chromosome  that 
I  formerly  supposed  to  be  the  idiochromosome-bivalent,  despite 
its  peripheral  position,  and  despite  the  fact,  which  I  had  myself 
described,  that  a  similar  small  chromosome,  also  peripheral  in  posi- 

JOUHNAL  OF  MORPHOLOGT,  VOL.  22,  NO.  I 


74 


EDMUND    B.    WILSON 


EXPLANATION    OF   TEXT    FIGURES 

Figures  1  to  9  are  from  camera  drawings,  and  are  not  schematized  except 
that  in  a  few  instances  the  chromosomes  have  been  artificially  spread  out  in  a 
series  in  order  to  facilitate  comparison.  Figs.  2  k-l  are  somewhat  more  enlarged 
than  the  others.  In  all  the  figures  d  denotes  the  double  chromosome  or  'd-chro- 
mosome,'  s  the  small  chromosome,  X  the  large  idiochromosome  and  Y  the  small. 


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Fig.  1  The  second  spermatocyte-division  in  Nezara  viridula.  a-d,  metaphases 
in  side  view;  e-g,  anaphases;  h,  i,  polar  views  of  two  sister-groups,  middle  ana- 
phase,  from  the  same  spindle  and  in  the  same  section. 

tion,  appears  in  several  other  pentatomids  (e.g.,  in  Euschistus,  Coe- 
nus  and  Mineus).  But  Nezara  forms  no  exception  to  the  rule  that 
the  central  chromosome  is  the  idiochromosome-bivalent.  In  N. 
viridula  this  is  immediately  apparent  in  side  views  (often  also  in 
polar  views)  where  the  central  chromosome  is  seen  to  consist  of  two 
very  unequal  components,  the  smaller  being  not  more  than  one 
fourth  or  one  fifth  the  size  of  the  larger  (fig.  1  a-c).  In  the  ana- 
phases  these  separate  and  pass  to  opposite  poles,  while  all  the  others 
divide  equally  (fig.  1  e-g).  Polar  views  of  middle  or  rather  late 
anaphases,  when  both  daughter-groups  can  be  seen  superposed 
in  the  same  section,  clearly  show  the  marked  difference  of  the 
two  groups  in  respect  to  the  idiochromosomes  (fig.  1  h-i).  All  the 
facts  are  here  so  nearly  similar  to  those  seen  in  Euschistus  or 
Lygaeus  as  to  require  no  further  description. 


STUDIES   ON   CHROMOSOMES  75 

In  N.  hilaris  the  conditions  differ  only  in  that  the  two  compo- 
nents of  the  central  chromosome  are  but  slightly  unequal;  but  in 
the  examination  of  at  least  two  hundred  of  these  divisions  I  have 
never  failed  to  detect  the  inequality.  A  series  of  side  views  are 
shown  in  fig.  2  a-i,  figs.  16-21,  two  of  which  show  all  the  chromo- 
somes. These  figures  illustrate  practically  all  the  variations 
that  have  been  seen  in  the  idiochromosomes.  The  most  charac- 
teristic condition  is  that  seen  in  2  a,  b,  d,  in  which  both  idiochro- 
mosomes (X  and  Y)  are  more  or  less  elongated  and  united  end  to 
end.  Less  often  one  of  them  assumes  a  more  spheroidal  form 
(fig.  2  e,  h,  i,  fig.  17).  The  size-difference,  though  always  evident, 
seems  to  vary  slightly  (perhaps  because  one  or  the  other  compo- 
nent may  be  more  or  less  compressed  laterally),  but  is  always  dis- 
tinctly greater  than  that  now  and  then  seen  in  other  bivalents. 

Fig.  2  j  shows  a  mid-anaphase2  (cf.  figs.  21-23)  in  which  the 
inequality  would  hardly  be  noticed  without  close  study  and  the 
comparison  of  other  cases.  Figs.  2  k  and  I  are  similar  stages 
showing  all  the  chromosomes  spread  out  in  a  series  for  the  sake 
of  comparison.  In  both,  the  two  idiochromosomes  are  easily 
distinguishable,3  and  the  larger  is  seen  to  be  one  of  the  three  largest 
chromosomes.  Figs.  2  w-n,  o-p,  q-r  and  s-t  are  pairs  of  sister- 
groups,  in  each  case  from  the  same  spindle  in  anaphase.  All  of 
these  are  selected  from  cases  in  which  a  distinct  size-difference 
appears  between  X  and  Y,  but  there  are  also  many  cases  in  which 
this  cannot  be  seen.  Such  a  case  was  figured  in  fig.  4  e-f  of  my 
first  '  Study'  the  correctness  of  which  is  confirmed  by  re-examina- 
tion of  the  original  section.  This  condition  is  due  simply  to  the 
fact  that  the  large  idiochromosome  is  more  elongated  than  the 
small,  so  that  the  size-difference  cannot  be  seen  in  polar  view; 
and  for  the  same  reason  it  is  often  not  evident  in  polar  views  of 
the  metaphase. 

2  This  and  the  two  following  figures  are  a  little  more  enlarged  than  the  others . 

3  Fig.  2 1  is  the  same  group  figured  in  fig.  4  d  of  my  first  'Study,'  carefully  redrawn 
and  corrected.     A  comparison  of  the  two  drawings  will  show  that  in  the  latter  a 
distinct  size-difference  between  X  and  Y  is  actually  shown  but  is  minimized  by 
the  fact  that  the  former  is  represented  a  trifle  too  small,  the  latter  a  little  too 
large.     It  is  now  also  evident  that  they  are  connected  by  two  connecting  fibres 
instead  of  by  one. 


76 


EDMUND    B.    WILSON 


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- 


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r 

Fig.  2  The  second  spermatocyte-division  in  Nezara  hilaris.  a-i,  metaphase 
figures  in  side  view,  o  and  e  showing  all  the  chromosomes;  j-l,  mid-anaphases;  in 
k  and  I  all  the  chromosomes  are  shown  artificially  spread  out  in  series;  m-n,  o-p, 
q-r,  s-t,  four  pairs  of  sister-groups  from  late  anaphases,  in  polar  view,  in  each  case 
from  the  same  spindle. 


STUDIES   ON   CHROMOSOMES  77 

b.  The  double  chromosome.  A  second  interesting  feature  of  the 
second  division  that  I  formerly  overlooked  is  the  presence  of  a 
remarkable  double  chromosome  which  in  the  metaphase  has  ex- 
actly the  appearance  of  a  butterfly  with  widespread  wings.  This 
chromosome  (which  may  be  called  the  d-chromosome)  is  shown  in 
profile  view  in  2  b-e  and  1  a-d,  16, 17,  20,  24,  25.  This  is  the  only 
chromosome  in  the  second  division  that  shows  any  approach  to  a 
quadripartite  form,  and  its  characters  are  so  marked  as  to  constitute 
the  most  striking  single  feature  of  the  division.  As  the  figures 
show,  it  is  one  of  the  largest  of  all  the  chromosomes.  It  always 
has  an  asymmetrical  tetrad  shape,  giving  exactly  the  appearance 
of  a  smaller  and  a  larger  dyad  in  close  union;  and  it  always  lies 
in  the  outer  ring,  so  placed  as  to  undergo  an  equal  division,  and 
with  the  larger  wings  of  the  butterfly  turned  towards  the  axis  of 
the  spindle.  In  polar  view  (3  j-rri)  the  duality  is  far  less  apparent 
and  sometimes  invisible,  even  upon  careful  focussing.  In  N. 
viridula  the  duality  is  always  apparent  in  side  view,  but  the  but- 
terfly shape  is  usually  less  evident  than  in  N.  hilaris. 

In  the  initial  anaphases  the  d-chromosome  divides  symmetri- 
cally, drawing  apart  into  two  bipartite  chromosomes  (2  j,  k,  1  g) ; 
but  this  is  seldom  evident  save  in  profile  view.  Viewed  from  the 
pole  the  duality  does  not  now  ordinarily  appear,  though  it  may 
still  sometimes  be  seen  upon  careful  focussing.  In  the  later  ana- 
phases  the  two  components  tend  to  fuse,  and  often  can  no  longer 
be  distinguished.  Not  seldom,  however,  the  duality  is  visible 
even  in  the  final  anaphases;  and  sometimes  this  is  so  conspicuous 
that  the  spermatid-group  seems  at  first  sight  to  comprise  eight 
instead  of  seven  separate  chromosomes  (n,  r,  s,  t). 

Since  the  duality  of  this  chromosome  does  not  certainly  appear 
in  the  spermatogonial  groups  or  in  the  first  spermatocyte-division, 
its  peculiar  form  in  the  second  division  might  be  supposed  to 
result  from  some  special  mechanical  relation  to  the  spindle-fibers 
in  that  division.  This  is,  however,  excluded  by  examination  of 
the  interkinesis,  in  which  the  chromosomes  are  irregularly  scat- 
tered. In  these  stages,  even  when  the  spindle  is  still  very  small 
and  the  chromosomes  lie  in  a  quite  irregular  group,  the  butterfly 
shape  is  already  perfectly  evident;  and  it  shows  no  constancy  of 


78  EDMUND    B.    WILSON 

relation  to  the  spindle-axis,  often  lying  at  right  angles  to  the 
latter.  Apparently  therefore  its  duality  arises  quite  independ- 
ently of  the  spindle  or  astral  rays,  and  its  constant  position  in 
the  fully  formed  spindle  is  the  result  of  a  later  adjustment.  In 
this  species,  as  in  many  others,  each  chromosome  is  connected  with 
the  pole  by  a  bundle  of  delicate  fibers.  In  case  of  the  d-chromo- 
some  this  bundle  is  very  broad,  but  I  cannot  be  sure  that  it  is 
double. 

At  first  sight  any  observer  would,  I  think,  take  the  d-chromo- 
some  to  be  merely  a  result  of  the  accidental  superposition  or  close 
adhesion  of  two  separate  dyads  of  unequal  size;  but  such  an  inter- 
pretation is  inadmissible.  When  all  the  chromosomes  can  be 
unmistakably  seen,  the  d-chromosome  is  found  to  constitute  one 
of  the  seven  separate  elements  invariably  present  in  this  division; 
and  since  the  diploid  number  is  14  in  both  sexes  this  chromosome 
must  represent  one  chromosome,  not  two,  of  the  original  sperma- 
togonial  groups.  It  is  certain,  therefore,  that  the  double  appear- 
ance does  not  result  from  close  apposition  of  two  separate  chromo- 
somes; it  is  therefore  not  a  "  tetrad"  in  the  ordinary  sense  of  the 
word — i.e.,  not  one  that  results  from  the  synapsis  of  two  chromo- 
somes that  are  originally  separate  in  the  diploid  groups. 

2.     The  first  spermatocyte-division 

This  division  requires  only  brief  mention.  As  stated,  it  shows 
eight  separate  chromosomes,  of  which  the  only  one  that  can  be 
positively  identified  is  the  Y-chromosome  of  N.  viridula.  This 
chromosome,  always  immediately  recognizable  in  this  species 
by  its  small  size  (3  c,  d,  f,  g,  i),  figs.  12,  13),  is  usually  central  in 
position,  like  the  m-chromosome  of  the  Coreidae,  but  this  is  not 
invariable.  Since  it  divides  equally,  and  without  association  with 
any  other  chromosome  (3  g)  it  is  evident  that  the  two  idiochro- 
mosomes  must  be  separate  and  univalent  in  this  division.  In  N. 
hilaris  (3  a,  b,  figs.  10,  11)  the  eight  chromosomes  usually  form  an 
irregular  ring,  there  is  no  central  chromosome,  and  neither  idio- 
chromosome  can  be  certainly  recognized.  It  nevertheless  seems 
a  safe  inference  from  what  is  seen  in  N.  viridula  that  the  two 
idiochromosomes  are  here  also  separate  and  univalent. 


STUDIES   ON    CHROMOSOMES 


79 


•  • 


a 


c 


XY- 


XY 


J 


k 


Fig.  3  First  and  second  spermatocyte-divisions  in  the  two  species  of  Nezara. 
a,  6,  first  division,  hilaris,  polar  views:  c,  d,  corresponding  views  of  viridula;  first 
division,  hilaris,  side  view  showing  five  of  the  chromosomes  in  position  and  the 
other  three  to  the  right  above;/,  corresponding  view  of  viridula;  g,  middle  ana- 
phase,  viridula,  showing  division  of  Y;  h,  first  division  metaphase,  hilaris,  all  the 
chromosomes  artificially  spread  out  in  series;  i,  corresponding  view  of  viridula; 
j,  k,  second  division  metaphase,  hilaris,  polar  views;  I,  m,  corresponding  views 
of  viridula. 


80  EDMUND    B.    WILSON 

In  this  division  the  d-chromosome  can  not  be  identified  in  either 
species.  Figs.  3  e,  /,  h,  i,  show  all  the  chromosomes  of  the  two 
species,  in  each  case  from  a  single  spindle  in  side  view.  Most 
of  them  have  a  simple  bipartite  form,  but  in  each  species  two  or 
three  of  them  often  appear  more  or  less  distinctly  quadripartite 
as  is,  of  course,  often  the  case  with  the  bivalents  in  this  division. 
In  N.  hilaris  one  of  the  largest  chromosomes  is  usually  more 
elongated  than  the  others,  and  each  half  shows  a  slight  trans- 
verse constriction.  I  suspect  that  this  may  be  the  d-chromosome, 
but  cannot  establish  the  identification. 

3.     The  growth-period  and  spermatocyte-prophases 

These  stages  fully  bear  out  the  conclusions  based  upon  the 
divisions  and  establish  the  identity  of  the  idiochromosome-pair 
with  the  chromatic  nucleolus  of  the  growth-period.  Throughout 
the  growth-period  each  nucleus  contains  a  single  intensely  stain- 
ing spheroidal  chromatic  nucleolus  and  in  addition  a  very  large, 
clearly  defined  pale  plasmosome,  which  is  sometimes  double. 
Series  of  drawings  of  these  two  bodies  (in  each  case  from  the  same 
nucleus,  and  in  their  relative  position)  are  given  in  figs.  4  i-l  and 
m-p,  from  cells  of  the  middle  growth-period.  They  are  also 
shown  in  figs.  26-29.  In  these  stages  no  sign  of  duality  is  to  be 
seen  in  the  chromatic  nucleolus,  even  after  long  extraction  or  in 
saffranin  preparations.  In  later  stages,  as  the  chromosomes  begin 
to  condense,  this  nucleolus  becomes  less  regular  in  outline,  and 
gradually  assumes  a  tetrad  form,  which  becomes  very  clear  as 
the  chromosomes  assume  their  final  shape.  This  transformation 
may  be  traced  without  a  break,  successive  stages  being  often  seen 
within  the  same  cyst.  Just  before  the  nuclear  wall  breaks  down 
this  tetrad  is  still  clearly  distinguishable  from  the  others  by  its 
asymmetrical  quadripartite  form,  as  seen  in  4  y,  z,  which  show  all 
the  chromosomes  (in  each  case  from  two  successive  sections). 
Figs.  4  q-t  show  four  views  of  this  tetrad  at  this  period  in  N. 
hilaris,  while  u-x  are  corresponding  views  of  N.  viridula.  These 
figures  (which  might  be  indefinitely  multiplied)  show  the  marked 
differences  between  the  two  species  in  respect  to  this  tetrad, 
obviously  corresponding  to  that  seen  between  the  idiochromosome 


•  *  9 


V      W 


Fig.  4  The  diploid  groups,  nucleoli  of  the  growth-period,  and  late  prophase- 
figures  of»the  two  species  of  Nezara.  a,  b,  spermatogonial  groups,  hilaris;  c,  d,  the 
same,  viridula;  e,  f,  ovarian  groups,  hilaris;  g,  h,  the  same,  viridula;  i-l,  chromatic 
nucleolus  and  plasmosome  from  the  growth-period,  in  each  case  from  the  same 
nucleolus  in  their  relative  position;  m-p,  corresponding  views,  viridula;  q-t,  the 
idiochromosome-tetrad  (chromatic  nucleolus)  from  prophase  nucleoli,  hilaris; 
u-x,  corresponding  views,  viridula;  y,  late  prophase  nucleus,  showing  all  the 
chromosomes,  hilaris  (combination  figure  from  two  sections) ;  z,  corresponding 
stage,  viridula,  three  of  the  chromosomes  from  adjoining  section  at  the  right. 


82  EDMUND   B.    WILSON 

bivalents  of  the  two  in  the  second  division.4  The  two  species 
may  in  fact  readily  be  distinguished  by  mere  inspection  of  the 
chromatic  nucleolus  at  this  period.  Already  at  this  time  the  two 
components  are  here  and  there  seen  to  be  separating,  but  as  a 
rule  they  do  not  finally  move  apart  until  the  nuclear  wall  has 
dissolved.  From  this  time  forward  they  cannot  be  individually 
identified  with  exception  of  the  small  idiochromosome  of  N. 
viridula,  which  is  obvious  at  every  period. 

As  far  as  my  material  shows,  the  earlier  stages  of  the  idio- 
chromosomes  can  not  be  so  readily  traced  in  Nezara  as  in  some 
other  species,  and  the  chromatic  nucleolus  can  not  actually  be  fol- 
lowed backward  to  the  spermatogonial  telophases — as  can  be 
done  in  such  forms  as  Lygaeus  or  Oncopeltus,  of  which  a  detailed 
account  will  be  given  in  a  later  publication.  The  prophase -figures, 
however,  decisively  establish  its  identity  with  an  unequal  pair  of 
chromosomes  that  divide  separately  in  the  first  spermatocyte- 
division;  and  in  N.  viridula,  one  of  these  is  certainly  the  small 
idiochromosome.  It  may  therefore  confidently  be  concluded 
that  the  chromatic  nucleolus  is  identical  with  the  idiochromo- 
some-pair,  as  in  so  many  other  cases.  Comparison  of  the  division- 
figures  proves  that  this  pair  can  not  be  identical  with  the  small 
pair  that  I  formerly  supposed  to  be  the  idiochromosome-pair; 
and  this  small  pair  is  moreover  usually  recognizable  in  the  pro- 
phase  groups  (s,  in  5  y,  z)  in  addition  to  the  unequal  pair. 

The  foregoing  facts  make  it  clear  that  in  Nezara  the  idiochromo- 
somes  undergo  a  process  of  synapsis  at  the  same  time  with  the 
other  chromosome-pairs,  and  that  their  separation  before  the  first 
division  is  a  secondary  process,  to  be  followed  by  a  second  conju- 
gation after  this  division  is  completed.  A  similar  process  often 
takes  place  in  many  other  Hemiptera.  There  are,  however,  some 
forms,  like  Oncopeltus,  in  which  the  idiochromosomes  are  always 
separate,  from  the  last  spermatogonial  division  through  all  the  suc- 
ceeding stages  up  to  the  end  of  the  first  division.  In  this  case, 
which  I  shall  describe  more  fully  hereafter,  there  can  be  no  doubt 
that  the  conjugation  which  follows  the  first  division  is  a  primary 
synapsis,  to  be  immediately  followed  by  a  disjunction. 

4  Cf.  the  earlier  figures  of  the  corresponding  tetrad  in  Brochymena  in  my  first 
'Study,'  fig.  7. 


STUDIES   ON   CHROMOSOMES  83 

4.     The  diploid  chromosome-groups 

In  these  groups  the  interest  centers  again  in  the  identity  of  the 
idiochromosomes  and  the  d-chromosome.  Of  the  14  separate 
chrosomomes  present  in  the  diploid  nuclei  of  both  sexes,  none 
shows  any  constant  indication  of  duality  (figs.  4  a-h).  The  d- 
chromosome  can  not,  therefore,  be  identified  in  these  stages. 
Secondly,  in  both  species  the  diploid  groups  of  the  two  sexes  show 
the  same  relation  as  in  other  Hemiptera  of  this  type,  though  this 
is,  of  course,  more  readily  seen  in  N.  viridula  than  in  hilaris,  owing 
to  the  small  size  of  the  Y-chromosome.  In  the  spermatogonial 
groups  of  this  species  (4  c,  d)  this  chromosome  is  at  once  recog- 
nizable while  in  the  female  groups  (g,  ti)  it  is  lacking,  its  place 
being  taken  by  one  of  larger  size.  In  both  sexes  the  small  pair 
(s,  s)  is  also  recognizable.  In  this  species,  accordingly,  the  Y- 
chromosome  is  confined  to  the  male  line,  and  the  Y-class  of 
spermatozoa  must  be  male-producing,  as  in  other  forms. 

In  N.  hilaris  the  Y-chromosome  can  not  be  identified  (4  a,  6), 
but  the  relation  of  the  spermatozoa  to  sex-production  is  shown  in 
another  way,  though  less  unmistakably  than  in  N.  viridula.  As 
already  described,  the  large  idiochromosome  or  X-chromosome  is 
one  of  the  largest  three  chromosomes  seen  in  the  second  division. 
We  should  therefore  expect  to  see  five  largest  chromosomes  in 
the  male  diploid  groups.  This  is  clearly  apparent  in  figs.  4  a,  6, 
and  is  also  shown  in  the  corresponding  figures  of  N.  viridula 
(c,  d)  though  not  quite  so  clearly.  One  of  these  five  in  the  male 
should  be  the  X-chromosome;  and  if  the  usual  relation  of  the 
spermatozoa  to  sex  hold  true,  there  should  be  six  largest  chro- 
mosomes in  the  diploid  groups  of  the  female.  This  relation  actu- 
ally appears  in  nearly  all  cases,  and  is  shown  in  figs.  4  e,  f,  g,  h,  in 
each  of  which,  again,  the  small  pair  (s,  s)  may  be  recognized. 
Though  this  evidence  is  in  itself  less  convincing  than  that  afforded 
by  N.  viridula  (since  the  relation  can  not  always  be  made  out 
with  certainty)  it  is  fully  in  harmony  with  the  latter,  and  sustains 
the  same  conclusion.5 

5  This  relation  is  shown  in  my  original  figures  of  N.  hilaris,  though  not  quite 
as  clearly  as  in  the  groups  here  figured.  In  my  first  'Study'  ('05)  the  five  largest 
chromosomes  are  very  clearly  shown  in  fig.  4  h,  and  are  also  evident  in  4  q.  In  the 
third  'Study'  the  relation  is  hardly  evident  in  the  male  but  fairly  so  in  the 
female  (figs.  5  I,  TO). 


84  EDMUND    B.    WILSON 

GENERAL 

5.     The  idiochromosomes 

The  case  of  Nezara  shows  how  readily  a  morphological  dimorph- 
ism of  the  spermatid-nuclei  may  be  overlooked  when  the  X-  and 
Y-chromosomes  do  not  differ  markedly  in  size ;  and  it  emphasizes 
the  necessity  for  the  closest  scrutiny  of  these  chromosomes  in  the 
study  of  this  general  question.  In  my  fourth  'Study'  I  placed 
with  Nezara  hilaris,  as  a  second  example  of  my  original  'third 
type/  the  lygaeid  species  Oncopeltus  fasciatus  (Dall.),  on  the 
strength  of  Montgomery's  account  of  the  conditions  in  the  male 
('01,  '06)  and  my  own  unpublished  observations  on  both  sexes. 
While  I  have  carefully  re-examined  this  case  also,  I  am  not  yet 
prepared  to  express  an  unqualified  opinion  in  regard  to  it.  Cer- 
tainly, in  very  many  of  the  cells  of  this  species,  at  every  period  of 
the  spermatogenesis,  the  idiochromosomes  (which  are  always  sep- 
arate up  to  the  second  division)  seem  to  be  perfectly  equal.  A 
slight  inequality  may  indeed  be  seen  in  some  cases ;  but  as  far  as  I 
can  yet  determine  this  seems  to  fall  within  the  range  of  the  size- 
variation  in  other  chromosomes  and  gives  no  positive  ground  for 
the  recognition  of  a  morphological  dimorphism  in  the  spermatozoa. 
A  similar  condition  has  been  described  in  several  other  insects,  not- 
ably in  some  of  the  Lepidoptera  (Stevens,  '06 ;  Dederer,  '08 ;  Cook, 
'  10) ,  in  the  earwig  Anisolaba  (Randolph,  '08)  and  apparently  also 
in  the  beetle  Hydrophilus  according  to  Arnold  ('08).  I  see  no  rea- 
son to  question  these  observations;  but  the  interpretation  to  be 
placed  on  them  is  by  no  means  clear  at  present.  The  experimental 
evidence  on  the  Lepidoptera  seems  to  demonstrate  that  in  at  least 
one  case — that  of  Abraxas  according  to  Doncaster  and  Raynor,— 
it  is  the  eggs  and  not  the  spermatozoa  that  are  sexually  dimorphic ; 
that  is,  in  the  terms  that  I  have  recently  suggested  ('10a),  in 
this  case  it  is  the  female  that  is  sexually  'digametic'  while  the 
male  is  'homogametic.'  Baltzer's  careful  work  on  the  sea- 
urchins  ('09)  clearly  demonstrates  a  cytological  sexual  dimorphism 
in  the  mature  eggs  of  these  animals,  and  shows  that  the  sperm- 
nuclei  are  all  alike.  In  cases,  therefore,  where  no  visible  dimorph- 
ism of  the  spermatid-nuclei  is  demonstrable,  two  possibilities 


STUDIES   ON    CHROMOSOMES  85 

are  to  be  considered,  namely,  (1)  that  it  may  be  the  female  which 
(as  in  sea-urchins)  is  the  digametic  sex,  and  (2)  that  one  sex  or 
the  other  may  still  be  physiologically  digametic  even  though  this 
condition  is  not  visibly  expressed  in  the  chromosomes.  The 
first  of  these  possibilities  may  readily  be  tested  by  cytological 
examination  of  the  female  groups.  The  second  can  only  be 
examined  by  means  of  experiment,  and  especially  by  experiments 
on  sex-limited  heredity.  It  is  interesting  that  the  work  of  Don- 
caster  and  Raynor,  cited  above,  and  the  more  recent  one  of  Morgan 
on  Drosophila  ('10)  have  given  exactly  converse  results,  the  former 
demonstrating  a  sexual  dimorphism  of  the  eggs,  the  latter  of  the 
spermatozoa.  This  agrees  with  the  cytological  data,  as  far  as 
they  have  been  worked  out.  The  researches  of  Stevens  ('08, 10), 
on  the  Diptera  establish  the  cytological  dimorphism  of  the  sper- 
matozoa in  these  animals,  while  all  observers  of  the  Lepidoptera 
have  thus  far  failed  to  find  such  dimorphism  in  this  group.  It 
thus  becomes  a  very  interesting  question  whether  a  cytological 
dimorphism  of  the  mature  eggs  may  be  demonstrable  in  the 
Lepidoptera;  though  a  failure  to  find  it  would  in  no  wise  lessen 
the  force  of  the  experimental  data.  Physiological  differences  be- 
tween the  chromosomes  are  of  course  not  necessarily  accompanied 
by  corresponding  morphological  ones — indeed  such  a  correlation 
is  probably  exceptional. 

(1)  (a)  Composition  and  origin  of  the  XY-pair.  The  facts 
seen  in  Nezara  again  force  upon  our  attention  the  puzzle  of  the 
Y-chromosome  or  'small  idiochromosome.'  It  is  remarkable 
that  two  species  so  nearly  akin  as  N.  hilaris  and  N.  viridula  should 
differ  so  widely  in  respect  to  this  chromosome;  though  this  is 
hardly  so  surprising  as  the  fact  that  in  Metapodius  this  chromo- 
some, as  I  have  shown  ('09,  '10)  may  actually  either  be  present  or 
absent  in  different  individuals  of  the  same  species.  These  facts 
show,  as  I  have  urged,  that  although  the  Y-chromosome  shows  a 
constant  relation  to  sex  when  it  is  present,  it  can  not  be  an  essen- 
tial factor  in  sex-production.  As  the  case  now  stands  this  might 
be  taken  as  a  direct  piece  of  evidence  against  the  view  that  the 
idiochromosomes  are  concerned  with  sex-heredity.  Further,  as 
I  have  pointed  out  ('10)  in  Metapodius  the  introduction  of  super- 


86  EDMUND    B.    WILSON 

numerary  Y-chromosomes  into  the  female  has  no  visible  effect 
upon  any  of  the  characters  of  the  animal,  sexual  or  otherwise; 
and  this  might  be  urged  against  the  whole  conception  of  qualita- 
tive differences  among  the  chromosomes  and  of  their  determina- 
tive action  in  development.  It  is  especially  in  view  of  these 
and  certain  other  general  questions  that  I  wish  to  indicate  some 
of  the  many  possibilities  that  must  be  taken  into  account  in  the 
consideration  of  this  problem.  My  discussion  is  throughout 
based  upon  the  assumption  that  the  chromosomes  do  in  fact 
play  some  definite  role  in  determination,  and  that  they  exhibit 
qualitative  differences  in  this  respect.  I  do  not  hold  that  they 
are  the  exclusive  factors  of  determination;  though  it  is  often  con- 
venient, for  the  sake  of  brevity,  to  speak  of  them  as  if  they  were 
such. 

(2)  Cytologically  considered,  the  morphological  dimorphism 
of  the  spermatozoa  seems  to  have  arisen  by  the  transformation 
of  what  was  originally  a  single  pair  of  chromosomes  comparable 
to  the  other  synaptic  pairs.  We  have  at  present  no  information 
as  to  whether  the  members  of  this  pair  were  equal  or  unequal  in 
size ;  but  in  either  case  there  are  grounds  for  the  assumption  that 
its  two  members  differed  in  some  definite  way  in  respect  to  the 
quality  of  the  chromatin  of  which  they  were  composed.  This 
pair,  which  may  be  called  the  primitive  XY-pair,  has  undergone 
many  modifications  in  different  species,  but  without  altering  its 
essential  relation  to  sex.  In  the  insects  (Hemiptera,  Coleoptera, 
Diptera)  its  most  frequent  condition  is  that  of  an  unequal  pair,  con- 
sisting of  a  'large  idiochromosome'  or  'X-chromosome/  and  a 
"  small  idiochromosome"  or  '  Y-chromosome,'  the  latter  being  con- 
fined to  the  male  line,  while  the  former  appears  in  both  sexes — 
single  in  the  male  and  paired  in  the  female.  That  all  gradations 
exist  between  cases  where  X  and  Y  are  very  unequal  (as  in  many 
Coleoptera  and  Diptera  and  in  some  Hemiptera)  and  those  in  which 
they  are  nearly  or  quite  equal  (Mineus,  Nezara,  Oncopeltus)  gives 
some  ground  for  the  conclusion  that  in  the  original  type  the 
XY-pair  was  but  slightly  if  at  all  unequal. 

By  disappearance  of  the  free  Y-member  of  this  pair  has  arisen 
the  unpaired  odd  or  'accessory'  chromosome,  which  accordingly 


STUDIES   ON   CHROMOSOMES  87 

has  no  synaptic  mate.  This  condition  seems  to  have  arisen  in 
more  than  one  way.  It  is  almost  certain  that  in  many  cases  the 
Y-chromosome  has  disappeared  by  a  process  of  gradual  and  pro- 
gressive reduction  (as  indicated  by  the  graded  series  observed 
in  the  Hemiptera  (Wilson,  '056,  '06).  In  some  cases  (of  which 
Metapodius  is  an  example)  the  same  result  may  have  been  pro- 
duced suddenly  by  a  failure  of  the  idiochromosomes  to  separate 
in  the  second  spermatocyte-division  (Wilson,  '096).  A  third  pos- 
sibility, first  suggested  by  Stevens  ('06),  is  that  the  X-element 
may  have  separated  from  a  YY-pair  with  which  It  was  originally 
united.  This  possibility  seems  to  be  supported  by  recent  obser- 
vations on  Ascaris  megalocephala,  where  the  X-chromosome  is 
sometimes  fused  with  one  of  the  other  pairs,  sometimes  free 
(Edwards,  '10). 

(3)  We  have  as  yet  no  positive  knowledge  as  to  how  the  X- 
member  of  the  XY-pair  originally  differed,  or  now  differs,  from 
the  Y,  or  as  to  how  this  difference  arose — a  definite  answer  to 
these  questions  would  probably  give  the  solution  of  the  essential 
problem  of  sex.  There  are,  however,  pretty  definite  grounds  for 
the  hypothesis  that  the  X-member  contains  a  specific  'X-chroma- 
tin'  that  is  not  present  in  the  Y-member,  and  that  the  XY-pair 
is  heterozygous  in  this  respect.  If  this  be  so,  the  primary  sexual 
differentiation  is  therefore  traceable  to  a  condition  of  plus  or 
minus  in  this  pair,  accompanied  by  a  corresponding  difference 
between  the  nuclear  constitution  of  the  two  sexes.  (Cf.  Wilson, 
'10a.)  Further,  there  is  also  reason  for  regarding  the  heterozy- 
gous condition  of  this  pair  as  due  to  the  presence  of  the  X-chroma- 
tin  in  one  member  of  a  pair  which  is  (or  originally  was)  homozy- 
gous  in  respect  to  its  other  constituents.  The  latter  may  be 
called  collectively  the  'Y-chromatin';  and  we  may,  accordingly, 
think  of  the  XY-pair  as  being  essentially  a  YY-pair  with  one 
member  of  which  the  X-chromatin  is  associated.6  Both  the  X- 

6  This  suggestion  is  in  principle  the  same  as  one  earlier  made  by  Stevens  ('06, 
p.  54)  that  the  Y-chromosome  represents  "some  character  or  "characters  which 
are  correlated  with  the  sex-character  in  some  species  but  not  in  others,"  with  one 
member  of  which  the  X-chromosome  is  fused;  and  that  "a  pair  of  small  chromo- 
somes might  be  subtracted  from  the  unequal  pair,  leaving  an  odd  chromosome." 


88  EDMUND    B.    WILSON 

chromatin  and  the  Y  may  themselves  be  composite,  thus  giving 
the  possibility  of  many  secondary  modifications.  The  point  of 
view  thus  afforded  opens  many  possibilities  for  an  understanding 
of  sex-limited  heredity,  as  indicated  beyond. 

(6)  Modifications  of  the  X-element.  This  view  of  the  XY-pair 
is  based  upon  two  series  of  facts,  of  which  the  first  includes  the 
various  modifications  of  the  X-member  of  the  pair  seen  in  dif- 
ferent species.  It  is,  perhaps,  most  directly  suggested  by  a  study 
of  the  pentatomid  species  Thyanta  custator.  In  this  common  and 
widely  distributed  species  I  have  found  two  races,  which  thus  far 
can  not  be  distinguished  by  competent  systematists, 7  but  which 
differ  in  a  remarkable  way  in  respect  to  both  the  total  number 
of  chromosomes  and  the  XY-pair.  In  one  of  these  races  (which 
I  will  call  the  'A  form'),  widely  distributed  throughout  the  south 
and  west,  the  total  number  in  both  sexes  is  16,  and  the  XY-pair 
of  the  male  is  a  typical  unequal  pair  of  idiochromosomes,  exactly 
like  that  seen  in  many  other  pentatomids  (e.g.,  Euschistus, 
Coenus  or  Banasa) .  These  are  shown  in  fig.  5  a,  b,  their  mode 
of  distribution  being  the  usual  one.  The  second  race  (the  'B 
form')  is  thus  far  known  from  only  a  single  locality  in  northern 
New  Jersey.  It  differs  so  remarkably  from  the  A  form  that  I 
could  not  believe  the  observations  to  be  trustworthy  until  repeated 
study  of  material,  collected  in  four  successive  years,  established  the 
perfect  constancy  of  the  cytological  conditions  and  the  apparent 
external  identity  of  the  two  forms.  In  this  race  the  XY-pair  is 
represented  by  three  small  chromosomes  of  equal  size,  which  are 
always  separate  in  the  diploid  groups  and  in  the  first  spermato- 
cyte-di vision  (fig.  5i),  but  in  the  second  division  are  united  to 
form  a  linear  triad  series  (5  c,  d) .  This  group  so  divides  that  one 
component  passes  to  one  pole  and  two  to  the  other  (5  e,  Ji) ,  the 

7  I  am  indebted  to  Mr.  E.  P.  Van  Duzee  for  a  careful  study  of  my  whole  series 
of  specimens  of  both  races.  He  could  find  no  constant  differential  between  them. 
Additional  studies  of  this  material  are  now  being  made  by  Mr.  H.  G.  Barber. 

Addendum.  Since  this  paper  was  sent  to  press  Mr.  Barber,  after  prolonged 
study,  has  reported  his  conclusion  that  the  'A  form'  is  Thyanta  custator  of 
Fabricius,  while  the  'B  form'  is  probably  Thyanta  calceata  of  Say,  which  has 
long  been  regarded  as  a  synonym  of  former  species. 


STUDIES   ON    CHROMOSOMES 


89 


J 


Y 
ft 

t 
X 


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m 


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^  P  0 

r 

i'  4 


r 

i 

A: 

•"5. 

r 

Y 

\ 

«• 

X 

I 

•  •  * 


•  • 


•  » 
•  •  /' 


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X 


Fig.  5  Comparison  of  the  XY-group  in  various  Hemiptera.  (a-i  are  orig- 
inal; the  others  from  Payne.)  a,  6,  Thyanta  custator,  'A  form,'  second  division 
in  side  view;  c,  d,  corresponding  views  of  the  'B  form';  e-h,  anaphases  of  same; 
i,  polar  view  of  first  division  of  same;  j,  k,  metaphase  chromosomes,  second  divi- 
sion, Diplocodus  exsanguis;  i,  similar  view  of  Rocconota  annulicornis;  TO,  similar 
view  of  Conorhinus  sanguisugus;  n,  Sinea  diadema;  o,  Prionidus  cristatus;  p, 
Gelastocoris  oculatus;  q,  anaphase  chromosomes  of  the  same  species;  r,  the  XY- 
group,  from  the  second  division,  Achollamultispinosa;  s,  diagram,  slightly  modified 
from  Payne,  to  show  the  distribution  of  the  XY-components  in  the  second  divi- 
sion of  the  same  species. 

JOURNAL  OF  MORPHOLOGY,  VOL.  22,  NO.  1 


90  EDMUND    B.    WILSON 

latter  being  usually  in  close  contact  and  in  later  anaphases  some- 
times hardly  separable  (50),  though  now  and  then  all  three  compo- 
nents are  for  a  time  strung  separately  along  the  spindle  in  the 
early  anaphases,  so  that  no  doubt  of  their  distinctness  can  exist 
(5  /) .  Comparison  of  the  diploid  groups  of  the  two  sexes  shows 
that  those  of  the  male  contain  but  three  of  these  small  chromo- 
somes and  those  of  the  female  four,  the  total  respective  num- 
bers being  27  and  28  (instead  of  16  in  both  sexes,  as  in  the  A 
form). 

These  facts  make  it  perfectly  clear  that  one  of  the  small  chromo- 
somes in  the  male  passes  to  the  male-producing  pole,  and  therefore 
corresponds  to  the  Y-chromosome;  while  the  other  two,  taken  to- 
gether, represent  the  large  idiochromosome,  or  X-chromosome,  of 
the  A  form — precisely  as  in  the  reduvioids  the  single  X-chromosome 
of  Diplocodus  is  represented  by  a  double  element  in  Fitchia, 
Rocconota  or  Conorhinus  (Payne).  Had  we  no  other  evidence 
on  this  point  we  might  assume  simply  that  the  original  X-chro- 
mosome has  divided  into  two  equivalent  X-chromosomes.  But 
there  are  other  facts  that  give  reason  for  the  conclusion  that  the 
breaking  up  of  a  single  X-chromosome  into  separate  components 
means  something  more  than  this.  In  the  B  form,  as  in  Fitchia  or 
Rocconota  (fig.  5  I),  the  X-element  consists  of  two  equal  compo- 
nents, but  in  Conorhinus  the  two  components  are  always  of  un- 
equal size  (5  ra).  In  Prionidus  and  in  Sinea  there  are  three  equal 
components  (5  n,  o),  in  Gelastocoris  four  equal  ones  (5  p,  and 
in  Acholla  multispinosa  five,  of  which  two  are  relatively  large 
and  equal  and  three  very  small  (5  r,  s).  In  every  case  these  com- 
ponents, though  quite  separate  in  the  diploid  groups  (and  usually 
also  in  the  first  spermatocyte-division)  act  as  a  unit  in  the  second 
division,  though  not  fused,  and  pass  together  to  the  female-produc- 
ing pole  (Payne,  '09,  '10). 

In  the  foregoing  examples  the  X-element  is  accompanied  by  a 
synaptic  mate  or  Y-chromosome.  The  following  are  examples  of 
a  similar  breaking  up  of  the  X-element  into  separate  components 
when  such  a  synaptic  mate  is  missing.  In  Phylloxera  (Morgan) 
the  X-element  consists  of  two  unequal  components,  sometimes 
separate,  sometimes  fused  together.  In  Syromastes  (Gross, 


STUDIES   ON   CHROMOSOMES  91 

Wilson)  it  consists  of  two  unequal  components,  always  separate, 
in  the  diploid  groups  but  closely  in  contact  (not  fused)  in  both 
spermatocyte-divisions.  The  recent  work  of  Guyer  ('10)  indi- 
cates a  similar  condition  in  the  X-element  of  man.  In  Agalena 
(Wallace)  there  are  two  equal  components,  always  separate. 
Finally,  in  Ascaris  lumbricoides  (Edwards,  '10)  there  are  five  com- 
ponents, separate,  and  scattered  in  the  diploid  groups  but  closely 
associated  in  the  spermatocyte-divisions. 

In  all  these  cases  the  significant  fact  is  that  not  only  the  number 
but  also  the  size-relations  of  these  components  are  constant;  and 
in  many  of  these  forms  this  fact  may  be  seen  in  such  multitudes 
of  cells,  and  with  such  schematic  clearness,  as  to  leave  no  manner 
of  doubt.  It  seems  impossible  to  understand  this  series  of  phe- 
nomena unless  we  assume  that  the  single  X-chromosome  is  essen- 
tially a  compound  body — i.e.,  one  that  consists  of  different  con- 
stituents that  tend  to  segregate  out  into  separate  chromosomes. 
We  are  led  to  suspect,  further,  that  the  composition  of  the  X- 
element,  even  when  it  is  a  single  chromosome,  may  differ  widely 
in  different  species  because  of  its  great  variations  of  size  as  between 
different  species.  For  instance,  in  the  family  of  Coreidae  it  is 
in  some  cases  very  large  (Protenor),  in  others  of  middle  size  (Che- 
linidea,  Narnia,  Anasa),  in  others  one  of  the  smallest  of  the  chromo- 
somes (Alydus).  Similar  examples  might  be  given  from  other 
groups. 

In  the  case  of  Thyanta,  therefore,  it  seems  a  fair  assumption 
that  the  double  X-element  of  the  B  form  likewise  represents  at 
least  a  partial  segregation  of  the  X-chromatin  from  other  con- 
stituents; and  the  latter  may  plausibly  be  regarded  as  represent- 
ing the  'Y-chromatin'  of  the  original  X-member  of  the  pair. 
In  other  words,  we  may  think  of  the  triad  element  as  a  YY-pair, 
one  member  of  which  is  accompanied  by  a  separate  X-chromosome. 
In  accordance  with  this  its  formula  should  be  X.Y.Y,  while  that 
of  the  A  form  is  XY.Y;  and  this  may  also  be  extended  to  other 
forms  of  similar  type.  If  this  be  admissible,  the  male  formula,  as 
regards  essential  chromatin-content,  becomes  in  general  XY.Y 
and  the  female  XY.XY,  both  sexes  being  homozygous  for  the 
Y-constituents,  while  in  respect  to  X  the  male  is  heterozygous, 


92 


EDMUND   B.    WILSON 


the  female  homozygous.  The  puzzle  of  the  Y-chromosome 
would  thus  be  solved;  for  although  a  separate  Y-chromosome, 
when  present,  is  confined  to  the  male  line,  its  disappearance 
only  reduces  the  male  from  a  homozygote  to  a  heterozygote  in 
respect  to  the  Y-chromatin,  and  the  introduction  of  supernumer- 
ary Y-chromosomes  into  the  female  (as  in  Metapodius)  brings  in 
no  new  element. 


L 


a 


A 

I 


Fig.  6  Compound  groups  formed  by  union  of  the  X-chromosome  with  other 
chromosomes  in  the  Orthoptera.  (a  and  b,  from  Sin6ty,  the  others  from  McClung. ) 
a,  triad  group,  first  division  of  Leptynia,  metaphase;  b,  division  of  similar  triad  in 
Dixippus;  c,  triad  group  formed  by  union  of  the  X-chromosome  with  one  of  the 
bivalents,  first  spermatocyte-prophase,  Hesperotettix;  d,  the  same  element  from 
a  metaphase  group;  e,  the  same  element  in  the  ensuing  interkinesis ;  /,  the  com- 
pound element  of  Mermiria,  from  a  first  spermatocyte  prophase;  g,  the  same  ele- 
ment in  the  metaphase  (now,  according  to  McClung,  united  to  a  second  bivalent 
to  form  a  pentad) ;  h,  the  same  element  after  its  division,  in  the  ensuing  telophase. 


The  same  general  view  as^hat  outlined  above  is  suggested  by  the 
constant  relation  known  to  exist  in  some  cases  between  the  X- 
chromosome  and  a  particular  pan- of  the  'ordinary  chromosomes/ 
The  first  observed  case  of  this  was  recorded  by  Sinety  ('01)  in 
the  phasmid  genera  Leptynia  and  Dixippus  (fig.  6  a,  b),  where  the 
X-chromosome  is  always  attached  to  one  of  the  bivalents  in  the 


STUDIES   ON   CHROMOSOMES  93 

first  spermatocyte-division,  and  passes  with  one  half  of  the  bivalent 
to  one  pole.  Since  the  spermatogonial  number  in  Leptynia  (36) 
is  an  even  one  and  twice  that  of  the  separate  chromosomes  present 
in  the  first  spermatocyte-division,  it  may  be  inferred  that  the 
X-element  is  already  united  with  one  of  the  ordinary  chromo- 
somes in  the  spermatogonia,  though  Sinety  does  not  state  this. 
Somewhat  later  McClung  ('05)  discovered  essentially  similar  rela- 
tions in  the  grasshoppers  Hesperotettix  and  Anabrus  (fig.  6, 
c-e),and  in  case  of  the  first  named  form  was  able  to  establish  the 
important  fact  that  it  is  always  the  same  particular  bivalent  with 
which  the  X-chromosome  is  thus  associated.  In  respect  to  the 
intimacy  of  this  association,  a  progressive  series  seems  to  exist, 
since  in  Leptynia  it  seems  to  take  place  in  the  spermatogonia,  in 
Hesperotettix  only  in  the  prophases  of  the  first  spermatocyte- 
division,  while  in  Thyanta  the  union  is  only  effected  after  the 
first  division  is  completed. 

Finally,  the  recent  observations  of  Boring  ('09),  Boveri  ('09) 
and  Edwards  ('10)  seem  to  establish  the  fact  that  in  Ascaris  megalo- 
cephala  the  X-element,  whether  in  the  diploid  groups  or  in  the 
maturation-divisions,  may  either  appear  as  a  separate  chromo- 
some (which  has  the  usual  behavior  of  an  accessory  chromosome) 
or  may  be  indistinguishably  fused  with  one  of  the  ordinary  chromo- 
somes. 

These  relations  may,  of  course,  be  the  result  of  a  secondary 
coupling;  and  I  myself  formerly  so  interpreted  them  ('09c).  But 
in  view  of  what  is  seen  in  Thyanta  or  the  reduvioids  we  may 
well  keep  in  mind  the  possibility  that  they  are  expressions  or 
remnants  of  a  more  primitive  association,  like  that  which  I  have 
assumed  for  an  original  XY-pair.  Whatever  be  their  origin,  the 
effect  is  the  same — a  definite  linking  of  the  X-chromatin  with 
that  of  one  of  the  other  pairs. 

Fig.  7  shows,  in  purely  schematic  form,  the  general  conception 
of  these  relations  that  has  been  suggested  above,  the  X-chromatin 
being  everywhere  represented  in  black.  A  is  the  primitive  XY- 
pair  from  which  all  the  other  types  may  have  been  derived.  By 
simple  reduction  of  such  a  pair  arises  the  ordinary  or  typical 
idiochromosome-pair  (B) ;  and  from  either  A  or  B  may  be  derived 


94 


EDMUND    B.    WILSON 


the  other  types  (C-G),8  or  the  more  complicated  ones  shown  in 
fig.  5.  I  represents  the  possible  mode  of  separation  of  the 
X-element  from  a  YY-pair,  as  suggested  by  Stevens;  and  this 
may  be  realized  in  Ascaris  megalocephala  (H).  J  and  K  are 
schemes  of  the  relations  seen  in  Hesperotettix,  Anabrus  and 
Mermiria  (cf.  fig.  6).  These  may  be  direct  derivatives  of  a 
primitive  XY-pair,  as  the  diagram  suggests,  or  may  be  a  result 


C.    Thyanta 
X 


XY-Pair 


I 


Idiochromosome 
Pair 

Y 


,1  I 

T.?        I 


D.   Fitchia 


£..    Conorhinus 
X 


G.  Syrornastes 


Mermiria, 


/i.Ascaris 


'  Jfesjberofattix 


Fig.  7  Diagram  illustrating  the  possible  relation  of  the  various  types  of  idio- 
chromosomes  to  a  primitive  XY-pair.  Explanation  in  text. 

of  secondary  coupling  of  X  with  other  elements.  In  either  case 
X  may  itself  have  such  a  composition  as  is  indicated  in  F  (Prote- 
nor). 

(c)  Sex-limited  heredity.  (1)  The  foregoing  considerations  have 
an  important  bearing  on  the  problem  of  sex-limited  heredity, 
for  they  give  us  a  very  definite  view  of  how  such  heredity  may  be 
effected.  It  is  not  my  intention  to  consider  this  subject  in  ex- 


8  These  figures  are  not  intended  to  indicate  the  precise  mode  of  segregation  of 
the  X-  and  Y-chromatins  of  the  X-element,  but  only  illustrate  possible  modes. 


STUDIES   ON   CHROMOSOMES  95 

tenso;  but  I  wish  to  indicate  some  of  the  possibilities  that  have 
been  opened  by  the  cytological  results,  even  at  the  risk  of  offering 
what  may  be  regarded  as  too  speculative  a  treatment  of  the  matter. 
It  is  obvious  that  any  recessive  mutation  should  exhibit  sex-limited 
heredity  when  crossed  with  the  normal  or  dominant  form,  if  it  be  due 
to  a  factor  contained  in  (or  omitted  from)  the  X-element.  For  in- 
stance, in  the  remarkable  Drosophila  mutants  discovered  by 
Morgan  ('10)  the  experimental  data  establish  the  fact  that  white 
eye-color  (which  seems  to  follow  the  same  type  of  heredity  as 
color-blindness  in  man)  is  linked  with  a  sex-determining  factor  in 
such  a  way  that  when  the  white-eyed  male  is  crossed  with  the 
normal  red-eyed  female,  the  former  character  is  neyer  transmitted 
from  father  to  son,  but  through  the  daughters  to  some  of  the 
grandsons  (theoretically  to  50  per  cent),  though  the  daughters  are 
not  themselves  white-eyed;  that  is,  after  such  an  initial  cross,  white 
eyes  fail  to  appear  in  the  Fi  generation  in  either  sex  and  in  the 
F2  generation  appear  only  in  some  of  the  males.  As  Morgan 
points  out,  this  follows  as  a  matter  of  course  if  the  factor  for  white 
eye  be  identical  with,  or  linked  with,  a  sex-determining  factor  in 
respect  to  which  the  male  is  heterozygous  or  simplex,  the  female 
homozygous  or  duplex.  The  X-element  exactly  corresponds  in 
mode  of  distribution  to  such  a  sex-determining  factor;  for  this 
chromosome,  too,  is  simplex  in  the  male,  duplex  in  the  female 
and  its  introduction  into  the  egg  by  the  spermatozoon  produces 
the  female  condition,  its  absence  the  male.  This  chromosome 
therefore,  as  I  have  shown  ('06),  is  never  transmitted  from  father 
to  son,  but  always  from  father  to  daughter.  Conversely,  the 
male  zygote  always  receives  this  chromosome  from  the  mother. 
So  precise  is  the  correspondence  of  all  this  with  the  course  of  sex- 
limited  heredity  of  this  type  that  it  is  difficult  to  resist  the  con- 
clusion that  we  have  before  us  the  actual  mechanism  of  such 
heredity — in  other  words,  that  some  factor  essential  for  sex  is 
associated  in  the  X-element  with  one  that  is  responsible  for  the 
sex-limited  character. 

This  will  be  made  clearer  by  the  accompanying  diagram  (fig. 
8)  where  the  X-element  assumed  to  be  responsible  for  a  recessive 
sex-limited  character  is  underscored  (X) .  This  character  may 


96 


EDMUND   B.   WILSON 


be  regarded  as  due  to  the  absence  of  some  particular  constituent 
that  is  present  in  the  normal  X-element. 


Fig.  8  Diagram  of  the  distribution  of  the  X-  and  Y-elements  in  successive 
generations,  illustrating  sex-limited  heredity.  The  underscored  X-element  (X) 
is  assumed  to  bear  a  factor  for  a  recessive  character  (e.g.,  white  eye-color),  while 
X  represents  the  normal  or  dominant  character  (e.g.,  red  eye-color).  Y  (being  the 
absence  of  X)  likewise  represents  the  recessive  character. 


Upon  pairing  the  affected  male  (XY)  with  the  normal  female 
(XX)  there  are  in  the  F,  generation  but  two  possible  combina- 
tions, XX  and  XY.  The  affected  X-chromosome  here  passes 


STUDIES   ON    CHROMOSOMES  97 

into  the  female,  and  the  male  is  normal ;  but  the  female  of  course 
likewise  shows  only  the  normal  (dominant)  character.  In  the 
following  F2  generation  (5)  there  are  four  possible  combinations 
XX,  XX,  XY  and  XY,  two  of  each  sex.  Though  X  is  present  in 
half  of  each  sex,  the  character  appears  only  in  the  males,  XY, 
again  because  of  its  recessive  nature.  By  crossing  together  males 
of  the  composition  XY  and  females  of  composition  XX,  some  of 
the  resulting  females  will  have  the  composition  XX,  and  the  sex- 
limited  character  is  thus  made  to  appear  in  the  female. 

When  the  female  is  the  heterozygous  or  digametic  sex — as  in 
sea-urchins,  in  Abraxas,  the  Plymouth  Rock  fowls,  etc. — exactly 
the  converse  assumption  has  to  be"  made.  Here,  as  Spillman 
('08)  and  Castle  ('09)  have  pointed  out,  the  observed  results 
follow  if  the  sex-limited  character  (e.g.,  lacticolor  color-pattern 
in  Abraxas)  be  allelomorphic  to,  or  the  synaptic  mate  of,  a  sex- 
determining  factor,  X,  that  is  present  as  a  single  element  in  the 
fe'male  but  absent  in  the  male.  The  formulas  now  become9 
(as  Spillman  has  indicated)  XG  (9  grossulariata),  GG  (cT  gross.) 
XG  (9  lacticolor)  and  GG  (d1  lact).  XG  X  GG  then  gives 
in  FI  XG  and  GG  (gross.  9  and  cf),  G  having  passed  from  the 
female  to  the  male.  The  following  cross,  XG  X  GG  gives  in  F2 
the  four  types  XG,  XG,  GG  and  GG, — i.e.,  grossulariata  appear- 
ing in  both  sexes  but  lacticolor  only  in  the  female.  By  crossing 
XG  with  GG  some  of  the  progeny  will  have  the  composition  GG 
(d"  lacticolor).  The  other  combinations  follow  as  a  matter  of 
course. 

This  interpretation  is  in  all  respects  the  exact  converse  of 
that  made  in  the  case  of  Drosophila,  which  is  also  the  case  with 

9  These  formulas  are  in  substance  the  same  as  those  stated  by  Mr.  Spillman  in 
a  private  letter  to  the  writer,  and  are  a  simplified  form  of  those  suggested  by  Castle 
('09).  The  interpretation  thus  given  seems  both  the  simplest  and  the  most  satis- 
factory from  the  cytological  point  of  view  of  all  those  that  have  been  offered.  It 
obviates  the  cytological  difficulties  that  I  urged  ('09)  against  Castle's  formulas, 
and  renders  unnecessary  the  secondary  couplings  that  I  suggested.  All  these 
ways  of  formulating  the  matter  conform,  of  course,  to  the  same  principle  and 
differ  only  in  details  of  statement.  Whether  the  synaptic  mate  of  X  is  directly 
comparable  to  the  Y-chromosome  of  other  insects  (in  which  case  the  female  formula 
becomes  XY  and  the  male  YY)  is  an  open  question. 


98  EDMUNP    B.    WILSON 

the  experimental  results,  as  Morgan  has  pointed  out.  It  seems 
probable  that  all  the  observed  phenomena  may  be  reduced  in 
principle  to  one  or  the  other  of  these  schemes,  though  many  modi- 
fications or  complexities  of  detail  probably  exist.  A  possible  basis 
for  many  such  modifications  seems  to  be  provided  by  the  cyto- 
logical  facts  already  known. 

(2)  We  might  assume  that  in  cases  of  the  first  type  (e.g.,  Droso- 
phila)  both  sex  and  the  characters  associated  with  it  are  deter- 
mined by  the  same  chromatin;  and  such  a  possibility  should 
certainly  be  borne  in  mind.  But  in  view  of  the  widely  different 
nature  of  the  characters  already  known  to  exhibit  sex-limited  hered- 
ity it  seems  improbable  that  we  can  regard  them  as  all  alike  due 
to  the  same  chromatin.  In  the  light  of  the  conclusions  that  have 
been  indicated  in  regard  to  the  composition  of  the  X-element,  it 
seems  more  probable  that  such  characters  may  be  determined  by 
the  various  other  forms  of  chromatin  ('  Y-chromatin')  associated 
with  the  X-chromatin.  If  these  constituents  be  identical  with 
those  contained  in  the  free  Y-chromosome  (the  synaptic  mate  of 
X)  sex-limited  heredity  would  of  course  not  appear,  since  the  two 
members  of  the  pair  would  be  homozygous  in  this  respect.  It 
should  make  its  appearance  as  a  result  of  the  dropping  out,  or 
other  modification,  of  certain  Y-constituents  of  the  X-element,  and 
such  a  mutation  might  arise  in  either  sex. 

We  may  perceive  here  the  possibility  of  understanding  many 
different  kinds  of  sex-limited  heredity,  perhaps  of  complex  types 
that  have  not  yet  been  made  known.  Such  a  possibility  is  sug- 
gested, for  example,  by  the  remarkable  relation  discovered  by 
McClung  ('05)  in  Mermiria  (fig.  Qf-h,  fig.  7  in  diagram),  where 
the  X-chromosome  is  in  the  first  spermatocyte-division  attached 
at  one  end  to  a  linear  chain  of  four  other  elements  to  form  a 
pentad  complex,  to  which  may  be  given  the  formula  XA .  ABB. 
This  so  divides  as  to  separate  XA  from  ABB.  The  interpretation 
to  be  placed  upon  this  is  a  puzzling  question  under  any  view,  and 
apparently  must  await  more  extended  studies  on  both  sexes,  per- 
haps also  on  other  forms,  before  it  can  be  fully  cleared  up.  Even 
here  the  possibility  exists,  I  think,  that  the  entire  complex  may 
have  arisen  by  the  differentiation  of  a  single  original  XY-pair; 


STUDIES   ON   CHROMOSOMES  99 

but  this  question  is  clearly  not  yet  ready  for  discussion.  How- 
ever such  associations  have  arisen,  the  result  is  equally  appli- 
cable to  the  explanation  of  sex-limited  heredity. 

(d)  Secondary  sexual  characters.  Castle  ('09)  has  offered  the 
interesting  suggestion  that  the  free  Y-chromosome  may  be  re- 
sponsible for  the  determination  of  secondary  sexual  characters 
in  the  male.  Though  I  have  criticized  this  view  ('09c)  I  now 
believe  it  may  be  true  for  certain  cases.  It  is  obviously  excluded 
when  the  Y-chromosome  is  missing;  and  since  nearly  related 
species — in  Metapodius  even  different  individuals  of  the  same  spe- 
cies— show  the  same  or  similar  secondary  male  characters  whether 
this  chromosome  be  present  or  absent,  it  seems  probable  that 
these  characters  are  in  general  determined  in  some  other  way. 
But  if,  as  I  have  suggested,  sex-limited  heredity  may  arise  through 
a  modification  of  the  Y-constituents  of  the  X-element,  it  follows 
that  the  YY-pair  thereby  becomes  heterozygous.  In  such  case, 
the  free  Y-chromosome,  being  confined  to  the  male  line,  should 
continue  to  represent  characters  that  are  no  longer  present  in 
the  female,  and  hence  would  be  indistinguishable  from  secondary 
male  characters  otherwise  determined.  It  has  further  become 
evident  (as  is  indicated  below)  that  the  chromosome-groups  are 
so  plastic  that  their  specific  composition  may  vary  widely  from 
species  to  species.  It  may  very  well  be,  therefore,  that  Castle's 
suggestion  may  apply  to  some  forms. 

6.     Modes  in  which  the  chromosome-number  may  change 

The  constant  and  characteristic  duality  of  the  'd-chromosome' 
in  the  second  division  suggests  a  series  of  questions  regarding  the 
mode  in  which  the  chromosome-number  may  change  that  have 
an  important  bearing  on  those  already  considered.  The  appear- 
ance of  this  chromosome  must  suggest  to  any  observer  that  it  is 
a  compound  body,  consisting  of  two  closely  united  components 
that  are  invariably  associated  in  a  definite  way;  but  it  is  especially 
noteworthy  that  its  duality  does  not  certainly  appear  before  the 
last  division.  This  case  must  be  added  to  the  steadily  increasing 
evidence  that  chromosomes  which  appear  single  and  homoge- 


100  EDMUND    B.    WILSON 

neous  to  the  eye  may  nevertheless  be  compound  bodies.  An 
important  part  of  it  is  derived  from  the  modifications  of  the  X- 
element  reviewed  above;  but  the  evidence  is  now  being  extended 
to  the  'autosomes'  or  ordinary  chromosomes  as  well.  The  double 
chromosome  of  Nezara  suggests  either  the  initial  stages  of  a  sep- 
aration of  one  chromosome  into  two  or  the  reverse  process — in 
either  case  that  we  have  before  us  one  way  in  which  the  number, 
and  the  composition,  of  the  chromosomes  may  change  from  species 
to  species.  This  is  supported  by  the  recent  observations  of 
Miss  E.  N.  Browne  ('10)  on  Notonecta.  In  N.  undulata  there 
are  always,  in  addition  to  a  typical  unequal  XY  pair,  two  small 
chromosomes  that  appear  in  all  the  divisions  as  separate  elements. 
In  N.  irrorata  there  is  always  but  one  such  chromosome,  the  total 
number  in  each  division  being  accordingly  one  less  than  in  N. 
irrorata.  N.  insulata  presents  a  condition  exactly  intermediate, 
there  being  sometimes  one  and  sometimes  two  such  small  chromo- 
somes. When,  however,  but  one  seems  to  be  present,  the  second 
may  often  be  seen  closely  adherent  to  one  of  the  larger  chromo- 
somes; and  the  latter  may  positively  be  identified,  by  its  size,  as 
always  the  same  one.  It  can  hardly  be  doubted,  as  the  author 
points  out,  that  we  here  see  three  stages  in  a  process  by  which 
the  chromosome-number  is  changing,  either  by  the  fusion  of  two 
originally  separate  chromosomes,  or  by  the  separation  of  one  into 
two.  It  is  of  the  utmost  importance  that  this  process  affects 
a  chromosome  that  can  be  positively  identified  as  the  same  in 
each  case;  for  this  proves  that  the  change  is  not  an  indefinite 
fluctation  but  the  expression  of  a  perfectly  orderly  process. 
While  there  is  here  (as  in  the  case  of  the  d-chromosome  of  Nezara) 
no  way  of  knowing  in  which  direction  the  change  is  taking 
place,  either  alternative  involves  the  conception  that  the  indivi- 
dual chromosomes  may  be  compound  bodies,  whether  as  a  re- 
sult of  previous  fusion  or  as  possible  starting  points  for  a  subse- 
quent segregation. 

The  facts  now  known  indicate  at  least  four  possible  ways  in 
which  the  chromosome-number  (and  in  three  of  these  also  the 
composition  of  the  individual  chromosomes,  may  change  from 
species  to  species. 


STUDIES   ON    CHROMOSOMES  101 

One  is  that  suggested  by  the  foregoing  phenomena,  i.e.,  the 
gradual  fusion  of  separate  chromosomes  into  one  or  the  reverse 
process. 

A  second  mode  may  be  the  gradual  reduction  and  final  disap- 
pearance of  particular  chromosome-pairs,  as  was  suggested  by 
Paulmier  ('99),  and  afterwards  by  Montgomery  and  myself,  in 
case  of  the  microchromosomes,  or  'm-chromosomes'  of  the  co- 
reid  Hemiptera.  In  respect  to  the  size  of  these  chromosomes,  a 
graded  series  may  be  traced  from  forms  in  which  they  are  very 
large  (as  in  Protenor)  through  those  where  they  are  of  intermediate 
size  down  to  cases  where  they  are  very  small  (as  in  Archimerus) 
and  finally  to  such  a  condition  as  that  seen  in  Pachylis  (fig. 
9  j-l)  where  they  are  almost  as  minute  as  centrioles  and  may 
almost  be  regarded  as  vestigial.  Four  of  these  stages  are  shown 
in  fig.  9.  In  Protenor  (a-c)  the  m-chromosomes  are  so  nearly 
of  the  same  size  as  the  next  smallest  pair  that  they  often  can  not 
be  positively  identified  in  the  spermatogonial  groups.  In  Lepto- 
glossus  phyllopus  (d-f)  they  are  always  recognizable,  though  not 
much  smaller  than  the  next  pair.  In  L.  oppositus  or  L.  corcu- 
lus  they  are  a  little  smaller.  In  Anasa  (the  form  in  which  they 
were  first  discovered  by  Paulmier)  they  are  of  middle  size  (g-i) , 
representing  perhaps  a  fair  average  of  the  group.  Several  other 
genera  (e.g.,  Metapodius)  show  intermediate  stages  between  this 
condition  and  that  seen  in  Archimerus  (figured  in  my  second 
'Study,'  and  more  recently  by  Morrill)  where  the  m-chromo- 
somes are  almost  as  small  as  in  Pachylis.  It  is  most  remarkable 
that  throughout  this  whole  series  the  m-chromosomes  exhibit 
essentially  the  same  behavior  (Wilson,  '056,  '06), usually  remain- 
ing separate  throughout  the  entire  growth-period  and  only  con- 
jugating in  the  final  prophases  of  the  first  spermatocyte-division, 
to  form  a  bivalent  which  with  rare  exceptions  occupies  the  center 
of  the  metaphase  group;  in  some  forms,  also  (e.g.,  Protenor,  Aly- 
dus)  they  show  a  marked  tendency  to  condense  at  a  much  earlier 
period  than  the  other  chromosomes.  The  m-chromosomes  of 
Pachylis,  excessively  minute  though  they  are,  exhibit  a  behavior 
in  all  respects  as  constant  and  characteristic  as  even  the  largest 
of  the  series.  In  the  Lygaeidae  they  seem  to  be  present  in  some 


102 


EDMUND    B.    WILSON 


X 


k 


Fig.  9  Comparison  of  the  m-chromosomes  in  Hemiptera.  (In  each  horizon- 
tal row  are  shown  at  the  left  a  spermatogonial  group,  in  the  middle  a  polar  view 
of  the  first  spermatocyte-division,  at  the  right  a  side-view  of  the  same  division.) 
a-c,  Protenor  belfragei;  d-f,  Leptoglossus  phyllopus;  g-i,  Anasa  tristis;  j-l, 
Pachylis  gigas. 


STUDIES   ON   CHROMOSOMES  103 

species  (Oedancala,  t.  Montgomery),  in  others  absent  (Lygaeus). 
In  the  Pyrrhocoridae  (Pyrrhocoris,  Largus)  they  are  absent  as 
far  as  known.  So  characteristic  is  the  behavior  of  these  chromo- 
somes as  to  leave  not  the  least  doubt  of  their  essential  identity 
throughout  the  whole  series;  and  this  series  may  be  regarded  as  a 
progressive  one,  in  one  direction  or  the  other,  with  the  same  reason 
as  incase  of  any  other  graded  series  of  morphological  characters. 
The  series  thus  shown  in  case  of  the  ra-chromosomes  is  as  gradual 
and  complete  as  in  case  of  the  Y-chromosome,  and  may  with 
the  same  degree  of  probability  be  regarded  as  a  descending  one. 
Thirdly,  it  is  probable  that  the  chromosome-number  may 
change  by  sudden  mutations  that  produce  extensive  redistribu- 
tions of  the  chromatin-substance  without  involving  any  commen- 
surate change  in  its  essential  content.  Were  gradual  changes, 
chromosome  by  chromosome,  the  usual  mode  of  modification, 
we  should  certainly  expect  to  find  such  conditions  as  are  seen  in 
Nezara,  in  Notonecta,  or  in  the  Coreidae,  more  frequently.  In 
some  groups,  however,  we  find  wide  differences  of  chromosome- 
number  between  species  even  of  the  same  genus,  and  even  be- 
tween those  that  are  very  nearly  related,  without  any  accompany- 
ing evidence  of  a  gradual  process  of  transition — for  instance, 
among  the  aphids  and  phylloxerans  (Stevens,  Morgan)  or  in  the 
heteropterous  genera  Banasa  and  Thyanta.  (Wilson,  '09d.)  In 
Banasa  dimidiata  the  diploid  number  is  16  in  both  sexes,  in  the 
nearly  related  B.  calva  26.  Of  the  two  races  of  Thyanta  custa- 
tor  described  above,  apparently  identical  in  other  visible  char- 
acters, one  has  in  both  sexes  the  diploid  number  16,  with  a 
simple  X-chromosome,  while  in  the  other  the  diploid  number  of 
the  male  is  27  and  that  of  the  female  28,  and  the  X-chromosome 
consists  of  two  components.  It  is  improbable  that  the  dif- 
ferentiation of  these  two  forms  has  been  accomplished  by  grad- 
ual modifications,  chromosome  by  chromosome.  It  seems  far 
more  likely  that  the  change  took  place  by  sudden  mutation,  invol- 
ving a  redistribution  of  the  nuclear  material  which  changed  its 
form  but  not  in  an  appreciable  degree  its  substance.  In  the  well 
known  case  of  Oenothera  gigas,  derived  by  sudden  mutation  from 
Oe.  Lamarckiana,  a  change  by  sudden  mutation  is  known  to  be 


104  EDMUND    B.    WILSON 


PLATE  1 

EXPLANATION   OF  FIGURES 

All  the  figures  from  photographs  of  sections.     Enlargement  1500  diameters. 

10,  11    First  spermatocyte-division  (N.  hilaris) 

12, 13    The  same  (N.  viridula) 

14,  15    Second  spermatocyte-division  (hilaris) 

16-25     Side  views  of  second  division  (hilaris) .     The  XY-pair  shown  in  16-23,  the 

d-chromosome  in  16,  17,  20,  24,  25;  the  small  chromosome  is  evident  in 

10,  12,  13,  14,  15,  17,  18. 

22  Initial  separation  of  X  and  Y 

23  Early  anaphase,  X  and  Y  separating  near  the  center  (hilaris) 

26-28  Nuclei  from  the  growth-period,  showing  chromosome-nucleolus  and  plas- 

mosome  (hilaris) 
29       Corresponding  stage  (viridula) 


STUDIES    ON    CHROMOSOMES 
EDMUND    B.    WILSON 


PLATE    1 


10 


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JOURNAL    OF    MORPHOLOGY,    VOL.    22,    NO.    1 


STUDIES   ON    CHROMOSOMES  105 

a  fact  (Lutz,  '07;  Gates,  '08),  though  it  may  be  due  in  this  instance 
to  a  simple  doubling  of  the  whole  group.  Such  cases  led  me  sev- 
eral years  ago  to  the  conclusion  "that  the  nucleus  consists  of 
many  different  materials  that  segregate  in  a  particular  pattern 
.  .  .  and  that  the  particular  form  of  segregation  may  readily 
change  from  species  to  species"  (Wilson,  '09d,  p.  2). 

Such  changes  must  involve  corresponding  ones  in  the  morpho- 
logical and  physiological  value  of  the  individual  chromosomes; 
and  we  must  accordingly  recognize  the  probability  that  these 
individual  values,  though  constant  for  the  species,  may  change 
from  species  to  species  as  readily  as  does  the  number.  Despite 
the  conformity  to  a  general  type  often  exhibited  by  particular 
genera  or  even  by  higher  groups,  the  individual  chromosomes  are 
therefore  per  se  of  subordinate  significance;  and  it  may  often  be 
practically  impossible  to  establish  exact  homologies  between  those 
of  different  species.  How  closely  this  bears  on  the  origin  of  the 
diverse  conditions  seen  in  the  composition  of  the  XY-pair  is 
obvious. 

Lastly,  it  is  almost  certain  that  changes  of  number  may  some- 
times arise  as  a  result  of  abnormalities  in  the  process  of  karyoki- 
nesis,  such  as  the  passage  of  both  daughter-chromosomes,  or  of 
both  members  of  a  bivalent,  to  one  pole.  In  Metapodius  I  found 
('096)  direct  evidence  of  this  in  case  of  the  XY-pair  itself,  and 
endeavored  to  trace  to  this  initial  cause  the  remarkable  variations 
of  number  that  occur  in  this  genus.  Many  other  observers  have 
recorded  anomalies  of  this  kind,  in  both  plants  and  animals.  It 
seems  entirely  possible,  as  has  been  suggested  by  McClung  ('05) 
and  by  Gates  ('08)  that  definite  mutations  may  be  traceable  to  this 
cause;  though  probably  such  abnormalities  may  in  general  be 
expected  to  lead  to  pathological  conditions. 

CONCLUSION 

Some  of  the  suggestions  offered  in  the  foregoing  discussion  are 
admittedly  of  a  somewhat  speculative  character;  but  they  are  not, 
as  I  venture  to  think,  mere  a  priori  constructions,  but  are  forced 
upon  our  attention  by  the  observed  facts.  The  time  has  come 

JOURNAL  OF  MORPHOLOGY,  VOL.  22,  NO.  1 


106  EDMUND   B.    WILSON 

when  we  must  take  into  account  more  fully  than  has  yet  been  done 
the  new  complexities  and  possibilities  that  have  continually  been 
unfolded  as  we  have  made  better  acquaintance  with  the  chromo- 
somes. In  this  respect  the  advance  of  cytology  has  quite  kept 
pace  with  that  of  the  experimental  study  of  heredity;  and  it  has 
established  so  close  and  detailed  a  parallelism  between  the  two 
orders  of  phenomena  with  which  these  studies  are  respectively 
engaged  as  to  compel  our  closest  attention. 

Studies  on  the  chromosomes  have  steadily  accumulated  evi- 
dence that  in  the  distribution  of  these  bodies  we  see  a  mechanism 
that  may  be  competent  to  explain  some  of  the  most  complicated 
of  the  phenomena  that  are  being  brought  to  light  by  the  study 
of  heredity.  Xew  and  direct  evidence  that  the  chromosomes 
are  in  fact  concerned  with  determination  has  been  produced  by 
recent  experimental  studies,  notably  by  those  of  Herbst  ('09) 
and  Baltzer  (J10)  on  hybrid  sea-urchin  eggs.  But  the  interest 
of  the  chromosomes  for  the  study  of  heredity  is  not  lessened,  as 
some  writers  have  seemed  to  imply,  if  we  take  the  view — it  is  hi 
one  sense  almost  self-evident — that  they  are  not  the  exclusive 
factors  of  determination.  Through  then"  study  we  ma}'  gain  an 
insight  into  the  operation  of  heredity .  that  is  none  the  less 
real  if  the  chromosomes  be  no  more  than  one  necessary  link  in  a 
complicated  chain  of  factors.  From  any  point  of  view  it  is 
indeed  remarkable  that  so  complex  a  series  of  phenomena  as  is 
displayed,  for  example,  in  sex-limited  heredity  can  be  shown  to 
run  parallel  to  the  distribution  of  definite  structural  elements, 
whose  combinations  and  recombinations  can  in  some  measure 
actually  be  followed  with  the  microscope.  Until  a  better  expla- 
nation of  this  parallelism  is  forthcoming  we  may  be  allowed  to  hold 
fast  to  the  hypothesis,  directty  supported  by  so  many  other  data, 
that  it  is  due  to  a  direct  causal  relation  between  these  structural 
elements  and  the  process  of  development. 

A  second  point  that  may  be  emphasized  is  the  remarkable  con- 
stancj'  of  the  chromosome-relations  in  the  species,  and  their  no 
less  remarkable  plasticity  in  the  higher  groups.  The  scepticism 
that  has  been  expressed  in  regard  to  constancy  in  the  species  finds, 
I  think,  no  real  justification  in  the  facts.  It  is  perfectly  true  that 


STUDIES   OX   CHROMOSOMES  107 

individual  fluctuations  occasionally  are  seen  in  the  number  of  the 
chromosomes,  in  the  process  of  synapsis,  in  the  distribution  of  the 
daughter-chromosomes,  and  hi  all  other  cytological  phenomena. 
It  is,  however,  also  true  that  most  observers  who  have  made  pro- 
longed, detailed  and  comparative  studies  of  any  particular  group, 
have  sooner  or  later  reached  the  conviction  that  hi  each  species 
all  the  essential  relations  in  the  distribution  of  the  chromosomes 
conform  with  wonderful  fidelity  to  the  specific  type.  So  true  is 
this  that  the  species  may  often  at  once  be  identified  by  an  expe- 
rienced observer  from  a  single  chromosome-group  at  any  stage  of 
the  maturation-process.  No  one,  I  believe,  who  has  engaged  for 
a  series  of  years  in  the  detailed  study  of  such  a  group,  for  instance, 
as  the  Hemiptera  or  the  Orthoptera,  returning  again  and  again  to 
the  scrutiny  of  the  same  material,  can  be  shaken  hi  the  convic- 
tion that  the  distribution  of  the  chromosomes  follows  a  perfectly 
definite  order,  even  though  disturbances  of  that  order  now  and 
then  occur.  But  it  is  equally  important  to  recognize  the  fact 
that  this  order  has  undergone  many  definite  modifications  of 
detail  from  species  to  species,  and  that  while  all  cases  exhibit  cer- 
tain fundamental  common  features,  we  cannot  without  actual 
observation  predict  the  particular  conditions  hi  any  given  case. 
It  is  now  evident  that  the  larger  groups  vary  materially  in  respect 
to  specific  conditions.  For  instance,  hi  the  orthopteran  family  of 
Acrididae  (McClung)  the  relations  seem  to  be  far  more 
uniform  than  such  a  group  as  the  Hemiptera,  where  great  spe- 
cific diversity  is  exhibited,  the  details  often  changing  from  species 
to  species  hi  a  surprising  manner — witness  the  species  of  Aphis 
or  Phylloxera  (Stevens,  Morgan),  those  of  Acholla  (Payne)  or 
of  Thyanta  (Wilson).  In  these  respects,  too,  the  cytologist  finds 
his  experience  running  parallel  to  that  of  the  experimenter  on 
heredity;  and  here,  once  more,  we  find  it  difficult  not  to  believe 
that  both  are  studying,  from  different  sides,  essentially  the  same 
problem. 

December  13,  1910. 


108  EDMUND   B.    WILSON 

LITERATURE  CITED 

ARNOLD,  G.  1908  The  nucleolus  and  microchromosomes  in  the  spermato- 
genesis  of  Hydrophilus  piceus.  Arch.  Zellforsch.,  vol.  2, 

BALTZER  1909  Die  Chromosomen  von  Strongylocentrotus  lividus  und  Echinus 
microtuberculatus.  Arch.  f.  Zellforsch.,  Bd.  2. 

1910  Ueber  die  Beziehung  zwischen  dem  Chromatin  und  der  Ent- 
wicklung  und  Vererbungsrichtung  bei  Echinodermenbastarden.  Habi- 
litationsschrift,  Wiirzburg.  Engelmann,  Leipzig. 

BORING  1909  A  small  chromosome  in  Ascaris  megalocephala.  Arch.,  f.  Zell- 
forsch., vol.  4. 

BOVERI,  TH.  1909  "  Geschlechtschromosomen"  bei  Nematoden.  Arch.  f.  Zell- 
forsch., Bd.  4. 

BROWNE,  E.  N.  1910  The  relation  between  chromosome-number  and  species 
in  Notonecta.  Biol.  Bull.,  vol.  20,1. 

CASTLE,  W.  E.     1909    A  Mendelian  view  of  sex-heredity.     Science,  n.  s.,  March  5. 

COOK,  M.  H.  1910  Spermatogenesis  in  Lepidoptera.  Proc.  Acad.  Nat.  Sci., 
Philadelphia,  April. 

DBDERER,    P.     1908    Spermatogenesis  in  Phyllosamia.     Biol.  Bull.,  vol.  13. 

EDWARDS,  C.  L.  1910  Theidiochromosomesin  Ascaris  megalocephala  and  Ascaris 
lumbricoides.  Arch.  f.  Zellforsch.,  vol.  5. 

GATES,  R.  R.  1908a  The  chromosomes  of  Oenothera.  Science,  n.  s.,  vol.  27, 
Aug.  2. 

1908b  A  study  of  reduction  in  Oenothera  rubrinervis.  Bot.  Gazette, 
vol.  46, 

1909  The  behavior  of  the  chromosomes  in  Oenothera  lata  x  O.  gigas. 
Ibid.,  vol.  48. 

GROSS,  J.  1904  Die  Spermatogenese  von  Syromastes  marginatus.  Zool.  Jahrb. 
Anat.  u.  Ontog.,  vol.  20. 

GTJYER,  M.     1910    Accessory  chromosomes  in  man.     Biol.  Bull.,  vol.  19. 

HERBST,  C.  1909  Vererbungsstudien,  VI.  Die  cytologischen  Grundlagen  der 
Verschiebung  der  Vererbungsrichtung  nach  der  mlitterlichen  Seite. 
Arch.  Entwicklungsm.,  Bd.,  27. 

LUTZ,  A.  M.  1907  A  preliminary  note  on  the  chromosomes  of  Oenothera  La. 
marckiana  and  one  of  its  mutants.  Sci.,  n.  s.  26. 

McCLUNG,  C.  E.  1905  The  chromosome  complex  of  orthopteran  spermatocytes. 
Biol.  Bull.,  vol.  9. 


STUDIES   ON    CHROMOSOMES  109 

MONTGOMERY,  T.  H.     1901    A  study  of  the  chromosomes  of  Metazoa.     Trans. 
Am.  Phil.  Soc.,  vol.  20. 

1906    Chromosomes  in  the  spermatogenesis  of  the  Hemiptera  Heterop- 
tera.     Trans.  Am.  Phil.  Soc.,  vol.  21. 

MORGAN,   T.   H.     1900a    A   biological  and  cytological  study  of  sex-determina- 
tion in  phylloxerans  and  aphids.     Jour.  Exp.  Zool.,  vol.  7, 

1910    Sex-limited  inheritance  in  Drosophila.     Science,  n.  s.  32,  July  22. 

MORIULL,  C.  V.     1910    The  chromosomes   in   the   oogenesis,    fertilization   and 
cleavage  of  coreid  Hemiptera.     Biol.  Bull.,  vol.  19. 

PAULMIER,  F.  C.     1899    The  spermatogenesis  of  Anasa  tristis.     Jour.  Morph., 
vol.  15,  Suppl. 

PAYNE,  F.     1909    Some  new   types  of  chromosome  distribution  and  their  rela- 
tion to  sex.     Biol.  Bull.,  vol.  16. 

1910    The  chromosomes  of  Acholla  multispinosa.     Biol.  Bull.,  vol.  18. 

RANDOLPH,  HARRIET.  1908  On  the  spermatogenesis  of  the  earwig,  Anisolaba 
maritima.  Biol.  Bull.,  vol.  15. 

SINETY,  R.  DE  1901  Recherches  sur  la  biologic  et  1'anatomie  des  phasmes.  La 
Cellule,  t.  19. 

SPILLMAN,  W.  J.  1908  Spurious  allemorphism.  Results  of  some  recent  investi- 
gations. Am.  Naturalist,  vol.  42. 

STEVENS,  N.  M.  1906  Studies  in  spermatogenesis,  II.  A  comparative  study  of 
the  heterochromosomes  in  certain  species  of  Coleoptera,  Hemiptera 
and  Lepidoptera,  etc.  Carnegie  Inst.  Pub.  36. 

1908  A  study  of  the  germ-cells  of  certain  Diptera,  etc.  Jour.  Exp. 
Zool.,  5,  3. 

1910    The  chromosomes  in  the  germ-cells  of  Culex.     Jour.  Exp.  Zool., 

vo!8. 

WALLACE,  L.  B.  1909  The  spermatogenesis  of  Agalena  nsevia.  Biol.  Bull., 
vol.  17. 

WILSON,  E.  B.  1905a  Studies  on  chromosomes,  I.  The  behavior  of  the  idiochro- 
mosomes  in  Hemiptera.  Jour.  Exp.  Zool.,  vol.  2. 

1905b  Studies  on  chromosomes,  II.  The  paired  microchromosomes, 
idiochromosomes,  and  heterotropic  chromosomes  in  Hemiptera.  Jour. 
Exp.  Zool.,  vol.  2. 

1906  Studies  on  chromosomes,  III.  The  sexual  differences  of  the 
chromosomes  in  Hemiptera.  Jour.  Exp.  Zool.,  vol.  3. 

1909a  Studies  on  chromosomes,  IV.  The  accessory  chromosome  in 
Syromastes  and  Pyrrhocoris.  Jour.  Exp.  Zool.,  vol.  6. 


110  EDMUND   B.   WILSON 

1909b  Studies  on  chromosomes,  V.  The  chromosomes  of  Metapodius, 
etc.  Jour.  Exp.  Zool.,  vol.  6. 

1909c  Secondary  chromosome-couplings  and  the  sexual  relations  in 
Abraxas.  Science,  n.  s.  29,  p.  748. 

1909d  Differences  in  the  chromosome-groups  of  closely  related 
species  and  varieties,  etc.  Proc.  Seventh  Internat.  Zool.  Congress, 
Aug.  1907. 

1910a  The  chromosomes  in  relation  to  the  determination  of  sex. 
Science  Progress,  no.  16,  April. 

1910b  Studies  on  chromosomes,  VI.  A  new  type  of  chromosome-com- 
bination in  Metapodius.  Jour.  Exp.  Zool.,  vol.  9. 

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C  ~  I 


Studies  on  Chromosomes 

VIII.  Observations  on  the  Maturation-Phe- 
nomena in  Certain  Heiniptera  and  Other 
Forms,  with  Considerations  on  Synapsis 
and  Reduction 


EDMUND  B.  WILSON 

From  the  Depurt  merit  of  /oology,  Columbia  University 


from  THK  J<n  KNAI,  nr  I^XCHKIMENTAL 
ZOOLOGT,  Vol.  \:\.  No.  :i.  Octoh.M-,  l'.U2 


RETURN  TO 
DIVISION  Of  GENETfCS 
HILGARD  HALL 


Reprinted  from  THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY,  VOL.  13,  No.  3, 
October,  1912 


STUDIES    ON    CHROMOSOMES 

VIII.    OBSERVATIONS    ON     THE     MATURATION-PHENOMENA     IN     CER- 
TAIN  HEMIPTERA   AND    OTHER    FORMS,    WITH    CONSIDERATIONS 
ON  SYNAPSIS  AND  REDUCTION 

EDMUND  B.  WILSON 

From  the  Department  of  Zoology,  Columbia  University 

NINE   PLATES 

CONTENTS 

Introduction 346 

I.  The  maturation-divisions  in  Oncopeltus  and  Lygaeus  with  reference  to 
the  sex-chromosomes 

1.  The  diploid  chromosome-groups 350 

2.  The  first  spermatocyte-division 351 

3.  The  interkinesis 354 

4.  The  second  spermatocyte-division 355 

5.  Size-relations  of  the  sex-chromosomes  in  Oncopeltus 357 

II.  The  growth-period 

1-.  Outline  of  the  stages 359 

2.  Stages  a  to  d.     The  pre-synaptic  stages.     Comparison   with   other 

insects . 362 

3.  Stage  e.     Synapsis,  synizesis 376 

4.  Stages/  and  g.     Post-synaptic  spireme  (pachytene,  diplotene),  and 

the  diffuse  or  confused  stage 377 

5.  Stages  h  to  j.     The  prophases 381 

6.  Comparative  considerations  regarding  the  growth-period 387 

7.  Comment  on  the  sex-chromosomes  in  Oncopeltus 390 

III.  Critical  considerations  on  the  maturation-phenomena  based  on  a  compari- 
son of    the   Hemiptera  with   Tomopteris,  Batracoseps  and  other 
forms 

1.  The  question  of  synapsis 391 

2.  The  question  of  the  reduction-division 407 

3.  The  chromosomes  and  heredity 419 


345 


THE   JOURNAL   OF   EXPERIMENTAL  ZOOLOGY,    VOL.    13,    NO.   3 
OCTOBER,    1912 


346  EDMUND  B.  WILSON 


In  this  paper  are  described  observations  on  certain  phases  of 
the  maturation-process  in  Oncopeltus  fasciatus  (Dall.),  Lygaeus 
bicrucis  -(Say),  and  some  other  Hemiptera,  together  with  the 
results  of  a  comparison  of  these  species  with  some  other  insects, 
with  Tomopteris  and  with  Batracoseps.  It  was  my  original 
object  to  clear  up  the  relations  of  the  sex-chromosomes  in  Onco- 
peltus and  to  trace  as  completely  as  possible  their  history  in  the 
maturation-process;  but  in  doing  this  it  was  found  necessary  to 
take  into  consideration  many  other  features  of  the  spermatogene- 
sis,  and  I  will  take  this  opportunity  to  present  some  conclusions 
based  on  a  broader  study  of  these  problems  on  which  I  have  long 
been  engaged. 

In  respect  to  the  sex-chromosomes,  Oncopeltus  is  of  especial 
interest  because  it  stands  on  the  border  line  between  species  in 
which  the  X-  and  F-chromosomes  are  visibly  unequal  in  size, 
and  in  which  a  corresponding  visible  difference  appears  between 
the  diploid  chromosome-groups  of  the  two  sexes,  and  those  in 
which  such  sexual  differences  can  not  be  seen.1  In  my  fourth 
'Study'  ('09  a)  Oncopeltus  was  classed  with  Nezara  hilaris  as 
an  example  of  the  latter  class  of  cases;  and  much  theoretic  impor- 
tance has  been  ascribed  to  both  these  forms  as  indicating  the  pos- 
sibility or  probability  that  the  spermatozoa  are  really  sexually 
dimorphic  even  when  no  visible  evidence  of  this  is  shown  by  the 
chromosomes.  In  my  seventh  'Study'  ('11  a)  )  I  showed,  con- 
trary to  my  original  account,  that  a  dimorphism  of  the  sper- 
matid-nuclei  is  in  fact  visible  in  Nezara;  but  in  regard  to  Oncopel- 
tus judgment  was  reserved  as  I  was  still  baffled  by  apparently 
contradictory  data.  I  am  now  in  a  position  to  clear  up  these 

1  The  first  account  of  Oncopeltus  was  given  by  Montgomery  ('01),  who  described 
the  sex-chromosomes  ('chromatin-nucleoli')  as  of  equal  size  in  the  male,  and  found 
that  they  remain  always  separate,  without  fusing  at  the  time  of  general  synapsis, 
and  divide  separately  in  the  first  spermatocyte-division.  Subsequently  ('06)  he 
added  that  these  chromosomes  (now  called  'diplosomes')  conjugate  to  form  a 
bivalent  after  the  first  division,  and  undergo  disjunction  in  the  second  division. 
In  my  fourth  'study'  ('09  a)  I  briefly  confirmed  these  accounts,  and  stated  that  the 
female  diploid  chromosome-groups  are  not  to  be  distinguished  by  the  eye  from  the 
male. 


STUDIES  ON  CHROMOSOMES  347 

contradictions  and  to  announce  a  definite  result.  Oncopeltus  is 
indeed  a  case  in  which  the  X-  and  Y -chromosomes  are  very  often 
sensibly  equal  in  size,  and  in  which  the  sexual  differences  of  the  dip- 
loid  groups  are  too  elusive  to  be  certainly  distinguished  by  the  eye. 
These  differences,  nevertheless,  almost  certainly  exist.  In  cer- 
tain individuals  a  distinct  size-difference  between  X  and  Y  is 
clearly  evident  in  a  large  percentage  of  the  cells  at  every  stage  of 
the  spermatogenesis ;  and  even  in  individuals  where  they  usually 
appear  equal,  inequality  is  unmistakably  seen  in  a  small  percent- 
age of  the  cells.  Aside  from  this,  the  close  similarity — almost 
identity — between  Oncopeltus  and  Lygaeus  bicrucis  in  all  other 
features  of  the  spermatogenesis  makes  it  extremely  probable  that 
the  same  essential  relations  of  the  chromosomes  to  sex  exist  in 
both,  though  they  are  only  clearly  obvious  in  Lygaeus. 

Like  many  other  insects  of  this  order,  Lygaeus  and  Oncopeltus 
are  distinctly  unfavorable  objects  for  the  direct  study  of  synapsis 
and  the  reduction-division — indeed  the  problem  of  synapsis  seems 
to  be  practically  insoluble  in  these  particular  forms.  They  never- 
theless present  some  very  interesting  features  for  comparison  with 
other  forms.  In  the  first  place,  the  history  of  the  sex-chromo- 
somes may  here  be  traced  with  almost  unique  clearness.  They 
may  be  identified  at  a  very  early  pre-synaptic  period,  and  followed 
thence  as  individual  bodies  through  every  later  stage  up  to  the 
time  of  their  final  delivery  to  the  spermatid-nuclei.  Every  step 
may  be  followed  in  their  conjugation  and  subsequent  disjunc- 
tion without  any  intervening  process  of  fusion.  In  case  of  these 
particular  chromosomes,  therefore,  I  consider  synapsis  and  dis- 
junction to  be  indisputable  facts.  It  is  far  otherwise  with  the 
ordinary  chromosomes  or  'autosomes.'  It  is  extremely  difficult 
to  gain  any  clear  idea  of  their  behavior  in  the  synaptic  period, 
and  I  fear  quite  impossible  to  trace  them  individually  through 
the  growth-period.  On  the  other  hand,  their  behavior  in  the 
pre-synaptic  period  and  in  the  maturation-prophases  exhibits 
some  very  interesting  features  when  compared  with  other  forms 
in  which  the  process  of  synapsis  and  its  sequel  are  more  accessible 
to  observation.  In  making  such  a  comparison  I  have  been  for- 
tunate in  the  opportunity  to  make  use  of  some  remarkably  fine 


348  EDMUND  B.  WILSON 

preparations  of  other  investigators,  to  whom  I  am  under  great 
obligations.  To  Professor  McClung  I  owe  the  loan  of  a  beautiful 
series  of  orthopteran  preparations,  especially  of  Phrynotettix, 
Mermiria,  Chortophaga  and  Achurum,  which  display  on  a  larger 
scale  some  of  the  same  phenomena  seen  in  the  pre-synaptic  stages 
of  the  Hemiptera,  and  leave  no  doubt  of  the  close  parallel  between 
the  two  groups  in  this  regard.  Even  more,  however,  I  am  in- 
debted to  Dr.  and  Madame  Schreiner,  and  to  Professor  Janssens, 
for  some  of  their  admirable  original  preparations  of  Tomopteris 
and  Batracoseps,  which. have  enabled  me  to  make  a  prolonged 
study  of  the  phenomena  of  synapsis  in  these  classical  objects. 
In  particular,  two  magnificent  slides  of  Batracoseps  by  Janssens 
demonstrate  both  the  complete  seriation  of  the  stages  and  the 
finest  details  of  the  nuclear  structures  with  incomparable  clear- 
ness. Though  I  have  also  made  many  preparations  of  this  form, 
as  well  as  of  Plethodon  and  other  Amphibia,  I  must  admit  my 
failure  to  equal  in  all  respects  the  standard  set  by  the  slides  of 
Janssens.  A  close  comparison  of  these  various  preparations  has 
more  than  ever  impressed  me  with  the  futility  of  attempting  the 
study  of  these  problems  with  material  that  is  unfavorable  for  the 
purpose,  or  with  prepar'ations  that  in  any  respect  fall  short  of  the 
highest  standard  of  technical  excellence.  Nothing  is  more  cer- 
tain than  that  different  objects  differ  enormously  in  the  clearness 
with  which  the  relations  are  displayed,  and  in  their  reaction  to 
fixing  and  staining  reagents.  Had  this  been  more  generally  recog- 
nized, many  erroneous  conclusions  and  some  ill-considered  criti- 
cism might  have  been  avoided. . 

Through  the  study  of  Batracoseps  and  Tomopteris  I  have  finally 
been  convinced — for  the  first  time,  I  must  confess,  as  far  as  the 
autosomes  are  concerned — (1)  that  synapsis,  or  the  conjugation 
of  chromosomes  two  by  two2  is  a  fact,  and  (2)  that  in  these  ani- 

2  A  number  of  writers  have  suggested  that  the  term  synapsis,  as  here  employed, 
should  be  abandoned  in  favor  of  some  less  ambiguous  word  (such  as  Haecker's 
term  'syndesis')  because  it  has  so  frequently  been  applied  to  the  contraction-fig- 
ure ('synizesis'  of  McClung).  I  am,  however,  in  favor  of  the  retention  of  the 
word,  for  the  ambiguity  has  arisen  simply  through  a  misunderstanding  of  Moore's 
meaning.  He  applied  the  term  'sj^naptic  phase,'  or  'synapsis,'  to  the  series  of 
changes  following  the  last  diploid  division  (during  the  'rest  of  transformation') 


STUDIES  ON  CHROMOSOMES  349 

mals  (perhaps  also  in  the  Orthoptera)  the  conjugation  is  a  side 
by  side  union,  or  parasynapsis.  On  the  other  hand,  the  evidence 
of  a  'reduction-division'  in  the  ordinary  use  of  the  term — i.e., 
the  disjunction  of  the  same  chromosomes  that  unite  in  synapsis — 
seems  to  me  to  be  far  short  of  a  demonstration.  In  these  forms 
synapsis  is  followed  by  a  union  so  intimate  that  no  adequate  evi- 
dence of  duality  can  for  a  time  be  seen  in  the  resulting  bivalents. 
I  do  not  for  this  reason  argue  against  the  conception  of  the  reduc- 
tion-division. On  the  contrary,  I  shall  offer  new  considerations 
in  favor  of  this  conception  in  a  somewhat  modified  form;  but  in 
case  of  the  autosomes  it  must  for  the  present  rest  mainly  upon 
indirect  evidence.  In  this  respect  the  autosomes  differ  notably 
from  the  sex-chromosomes,  at  least  in  the  male  sex;  and  this 
difference  may  be  of  significance  for  some  of  the  most  interesting 
phenomena  of  sex-heredity. 


in  the  course  of  which  the  apparent  number  of  chromosomes  is  reduced  to  one-half. 
"There  are  thus,  after  the  rest  of  transformation,  only  one  half  as  many  chromo- 
somes, i.  e.,  separate  chromatin-masses,  as  there  were  before,  and  the  halving  of 
their  number,  being  brought  about  while  the  nuclei  are  still  at  rest,  is  to  that 
extent  comparable  to  what  is  now  known  to  go  forward  during  the  maturation  of 
the  reproductive  elements  of  plants.  I  therefore  propose  the  term  Synaptic  Phase 
(from  avva-irru,  to  fuse  together)  to  denote  the  period  at  which  this  most  impor- 
tant change  appears  in  the  morphological  character  of  reproductive  cells"  ('95,  p. 
287.  In  subsequent  pages  the  phrase  '  synaptic  phase'  is  often  shortened  to  'synap- 
sis'in  the  same  sense).  This  'most  important  change'  is  obviously  the  halving 
of  the  number  of  chromosomes ;  and  nowhere  in  his  paper  is  the  word  applied 
to  the  contraction-figure,  though  the  latter  is  stated  to  be  "characteristic  of  this 
particular  phase  in  the  spermatogo-  and  ovogenesis  of  a  great  variety  of  animal 
forms"  (p.  305).  Though  there  was,  perhaps,  some  obscurity  in  his  original  use  of 
the  word,  all  doubt  as  to  Moore's  meaning  is  removed  in  a  later  paper,  published 
jointly  with  Farmer  ('05),  where  synapsis  is  precisely  denned  as  "that  series  of 
events  which  are  concerned  in  causing  the  temporary  union  in  pairs  of  pre-maiotic 
chromosomes"  (p.  490).  The  fact  that  so  many  later  writers  have  misapplied  it 
should  not  debar  us  from  the  continued  use  of  so  convenient  and  appropriate  a 
term — one  that  seems  particularly  fitting  if  it  be  a  fact,  as  a  number  of  excellent 
observers  have  concluded,  that  synapsis  is  followed  by  actual  fusion.  I  can  dis- 
cover no  reason  why  McClung's  term  'synizesis'  should  not  be  generally  employed 
for  the  contraction-figure,  as  it  already  is  by  most  American  writers. 


350  EDMUND  B.  WILSON 

I.  THE   MATURATION-DIVISIONS    IN    ONCOPELTUS   AND    LYGAEUS 
WITH    REFERENCE     TO    THE     SEX-CHROMOSOMES 

In  Oncopeltus  the  diploid  number  of  chromosomes  is  sixteen 
in  both  sexes  (figs.  1  to  5,  photos,  1,  2)  in  Lygaeus  fourteen  (fig.  6), 
and  the  second  spermatocyte-division  shows  half  these  numbers 
that  is,  eight  in  the  former  case,  seven  in  the  latter.  The  first 
division  shows  in  each  case  one  more  than  the  haploid  number, 
owing  to  the  fact,  repeatedly  described  heretofore  in  other  Hemip- 
tera,  that  in  the  first  division  the  X-  and  F-chromosomes  divide 
as  separate  univalents,  while  in  the  second  they  are  united  to 
form  a  bivalent.  In  both  Oncopeltus  and  Lygaeus  these  chro- 
mosomes conjugate  in  the  final  anaphase  of  the  first  division, 
just  as  the  cell  is  about  to  divide. 

1.  The  diploid  chromosome-groups 

The  spermatogonial  and  oogonial  divisions  require  but  brief 
description  since  they  present  no  striking  features  and  the  size- 
differences  are  but  slightly  marked.  In  Lygaeus  bicrucis  (fig.  6) 
the  fourteen  chromosomes  are  in  the  main  similar  to  those  of  L. 
turcicus  as  described  in  my  first  and  third  'Studies'  ('05,  '06) 
though  the  F-chromosome  is  relatively  smaller  in  the  latter  spe- 
cies. The  -XT-chromosome  can  not  be  identified  by  the  eye,  but 
must  be  at  least  twice  the  size  of  the  F-chromosome,  as  indicated 
by  the  maturation-divisions  and  by  the  spermatogonial  groups 
themselves.  Unfortunately  my  material  of  this  species  does  not 
show  a  single  good  equatorial  plate  in  the  female;  but  the  relations 
are  here  no  doubt  the  same  as  in  L.  turcicus. 

In  Oncopeltus,  of  which  I  have  abundant  material  of  both  sexes, 
the  size-differences  of  the  chromosomes  are  even  less  marked  than 
in  Lygaeus,  and  it  is  impossible  to  identify  pairs  of  different 
sizes.  Careful  study  fails  to  reveal  any  differences  between  the 
diploid  groups  of  the  two  sexes  that  are  sufficiently  marked  or 
constant  to  give  any  certain  result  (figs.  1  to  5).  fn  the  male  one 
chromosome  not  infrequently  is  somewhat  smaller  than  the  others 
(fig.  2),  and  this  may  be  the  F-chromosome;  but  very  often  this 
is  not  evident,  even  in  other  spermatogonia  from  the  same  cyst 
(figs.  1,  3).  As  will  be  shown  beyond,  the  X-  and  F-chromosomes 


STUDIES  ON  CHROMOSOMES  351 

are  often  somewhat  unequal  in  later  stages;  but  this,  too,  is  incon- 
stant. Oncopeltus  is  in  fact,  therefore,  a  form  in  which  the  sexual 
differences  of  the  chromosome-groups  are  too  slight  or  too  elusive 
to  be  distinguished  by  the  eye.  It  is,  however,  perfectly  certain 
that  an  XY-pair  is  present,  the  members  of  which  show  all  the 
characteristic  peculiarities  of  behavior  that  characterize  these 
chromosomes  in  other  forms. 

2.  The  ftrsl  spermatocyte-division 

The  maturation-divisions  are  shown  with  remarkable  clearness 
in  Oncopeltus  and  ^ygaeus — indeed,  either  of  them  might  be 
taken  as  a  model  of  those  Hemiptera  in  which  a  simple  XY-p&ir 
is  present.  The  following  account  applies  primarily  to  Oncopel- 
tus, Lygaeus  being  described  only  by  way  of  comparison. 

In  the  first  division  appear  nine  separate  chromosomes  in  a 
grouping  of  remarkable  constancy.  Seven  of  the  nine  bivalents 
are  grouped  in  an  irregular  ring,  near  the  center  of  which  lie  the 
univalent  X-  and  F-chromosomes,  side  by  side  but  not  in  contact 
(figs.  8,  9,  11,  photo.  3).  The  constancy  of  this  grouping  appears 
from  the  following  data.  Two  hundred  clear  polar  views,  taken 
at  random,  did  not  show  a  single  case  of  more  than  nine  chromo- 
somes; plus  variations  of  number  in  this  division  (such  as  are 
occasionally  seen  in  many  species)  must  therefore  be  very  rare — 
indeed,  I  have  never  seen  such  a  case.3  Of  the  two  hundred  cases, 
one  hundred  and  seventy- two  showed  the  grouping  just  described. 
In  the  remaining  twenty-eight  the  deviations  were  unimportant; 
the  ring  may  show  a  gap  at  "one  side  (figs.  10,  12),  one  of  the  biva- 
lents may  lie  inside  it  (fig.  10),  or  (rarely)  one  or  both  the  sex- 
chromosomes  may  lie  in  the  ring  (fig.  13).  In  only  two  cases  did 
the  sex-chromosomes  not  lie  side  by  side;  in  these,  they  were 
separated  by  one  bivalent  (fig.  13). 

The  size-relations  alone  sufficiently  indicate  that  the  two  small 
central  chromosomes  are  the  univalent  sex-chromosomes  (a  fact 

3  Apparent  minus  deviations  are  of  course  common,  but  are  disregarded  because 
evidently  due  in  most  (all?)  cases  to  the  fact  that  one  or  more  chromosomes  lie 
outside  the  plane  of  section. 


352  EDMUND  B.  WILSON 

fully  established  by  study  of  the  growth-period  and  the  pro- 
phases)  ;  for,  were  one  or  both  bivalent,  the  spermatogonial  groups 
should  show  one  or  two  corresponding  pairs,  which  is  not  the  case, 
In  Lygaeus  (fig.  7)  the  grouping  is  the  same,  but  only  six  bivalents 
are  present,  and  the  sex-chromosomes  are  conspicuously  unequal 
in  size. 

The  composition  of  these  chromosomes  is  better  seen  in  smears 
than  in  sections,  and  better  in  Protenor  (described  beyond)  than 
in  either  of  these  forms.  In  sections,  side  views  of  the  full  meta- 
phases  (figs.  14  to  17,  photo.  4)  usually  show  all  of  the  chromosomes 
as  simple  dumb-bell  figures,  though  indications  of  a  quadripartite 
form  sometimes  appear.  In  smears  the  bivalents  are  often  seen 
to  be  quadripartite,  owing  to  the  presence  of  a  longitudinal  split 
in  addition  to  the  transverse  constriction;  but  this  never  appears 
in  the  univalents.  All  the  chromosomes  alike  divide  'trans- 
versely'— that  is,  across  the  constriction  of  the  dumb-bell.  In 
case  of  the  bivalents,  therefore,  the  early  anaphase-chromosomes 
are  double  bodies,  while  the  sex-chromosomes  are  single,  but  this 
contrast  only  appears  clearly  in  smears,  owing  to  the  close  union 
of  the  two  halves.  In  this  respect  the  relations  are  less  clearly 
seen  in  these  forms  than  in  some  others,  such  as  Anax,  where  the 
anaphase-chromosomes  are  clearly  double  (cf.  Lefevre  and  Mc- 
Gill,  '08),  or  Aprophora,  where  the  same  condition  is  conspicuously 
shown  (Stevens,  '06). 

The  anaphases  are  of  particular  interest  because,  as  has  been 
mentioned,  a  conjugation  between  the  X-  and  F-chromosomes 
takes  place  in  the  later  stages.  As.  the  division  begins  and  the 
daughter-chromosomes  are  separating,  a  marked  contrast  in  form 
often  appears  between  the  sex-chromosomes  and  the  autosomes 
(figs.  19  to  21).  The  latter  are  more  or  less  extended  transversely 
and  often  show  a  slight  constriction,  thus  giving  evidence  of  their 
double  nature,  which  is  accentuated  by  the  very  conspicuous 
double  fibres  by  which  they  are  connected.  The  latter  are  so 
thick,  and  stain  so  deeply,  as  to  appear  as  if  spun  out  from  the 
chromosomes  themselves  (as  has  been  noted  by  other  observers). 
On  the  other  hand,  the  sex-chromosomes  do  not  show  such  a  con- 
striction, remaining  nearly  circular  in  outline,  while  the  connect- 


STUDIES  ON  CHROMOSOMES  353 

ing  fibers  are  much  less  conspicuous,  and  often  appear  single. 
Up  to  the  middle  anaphases  the  sex-chromosomes  remain  always 
separate  (figs.  18,  19).  In  the  later  anaphases  all  the  chromo- 
somes draw  more  closely  together,  and  often  come  more  or  less 
into  contact,  though  without  losing  their  original  grouping; 
but  in  case  of  the  autosomes  the  contact  is  but  casual  and  tem- 
porary, while  the  sex-chromosomes  become  definitely  attached 
to  each  other  to  form  a  dumb-bell. shaped  body  at  the  center  of 
the  group  (figs.  19,  21).  By  this  process  the  total  number  of  sepa- 
rate chromatin-elements  is  reduced  from  nine  to  eight  (the  hap- 
loid  number).  In  Lygaeus  the  process  takes  place  in  exactly  the 
same  way  and  may  be  seen  with  equal  clearness,  both  in  polar 
views  and  in  side-views.  In  both  species  polar  views  of  the  final 
anaphases  show  that  the  chromosomes,  save  for  their  more 
crowded  condition,  have  retained  the  same  grouping  as  in  the 
metaphase  (figs.  24  to  29),  the  XT-bivalent  being  at  the  center, 
surrounded  by  the  other  chromosomes  in  the  form  of  a  ring. 
These  facts  are  seen  so  clearly  and  in  so  great  a  number  of  cases 
as  to  remove  every  doubt  that  in  these  species  the  conjugation 
between  X  and  Y  regularly  takes  place  at  the  poles  of  the  spindle 
before  the  first  maturation-division  has  been  completed.4' 

In  Lygaeus  the  XF-bivalent  thus  formed  is  readily  distinguish- 
able by  the  inequality  of  its  two  components  (figs.  28,  29,  46). 
In  Oncopeltus  it  is  often  not  thus  marked  but  its  identity  is  no 
less  certainly  revealed  in  another  way.  As  the  figures  show,  the 
autosomes  still  show  but  slight  indication  of  a  transverse  constric- 
tion, and  can  hardly  be  described  as  dumb-bell  shaped  until  a 
later  period.  The  XF-bivalent,  on  the  other  hand,  is  invariably 
deeply  constricted,  so  as  to  have  a  conspicuously  dumb-bell  shape, 
and  it  often  still  appears  like  two  chromosomes  that  are  merely  in 
contact.  This  characteristic  difference  persists  throughout  the 
entire  interkinesis,  and  is  still  perfectly  obvious  in  the  ensuing 
metaphase  of  the  second  divi£ion. 


4  I  described  and  figured  this  process  in  the  case  of  Coenus  in  my  first  'Study' 
('05  b)  but  did  not  recognize  its  constancy.  I  now  incline  to  think  that  it  will 
be  found  to  occur  in  the  same  way  in  many  other  forms. 


354  EDMUND  B.  WILSON 

3.  The  interkinesis 

The  interkinesis  has  hitherto  been  very  briefly  treated  by  my- 
self and  other  observers  of  the  insects,  because  in  most  species  the 
chromosomes  are  so  closely  crowded  at  this  time  as  to  preclude 
accurate  study.  Lygaeus  (at  least  in  my  material)  is  no  excep- 
tion to  this,  but  Oncopeltus  fortunately  shows  every  stage  of  the 
interkinesis  with  remarkable,  clearness.  There  is  no  'resting 
stage'  between  the  two  divisions,  no  nuclear  vacuole  is  formed, 
and  both  the  chromosomes  and  the  centrioles  retain  their  individ- 
uality throughout. 

At  the  moment  when  the  equatorial  furrow  has  appeared  and 
the  conjugation  of  X  and  Y  has  taken  place,  the  centrioles  are 
already  rather  far  apart,  and  still  lie  at  some  distance  from  the 
chromosome-group  (fig.  22) .  All  the  achromatic  elements  are  now 
so  delicate  that  it  is  difficult  to  make  sure  of  the  exact  structure; 
but  it  is  certain  that  each  centriole  is  surrounded  by  a  small,  but 
very  distinct  aster,  and  the  two  seem  to  be  connected  by  a  delicate 
central  spindle.  As  the  cell  divides  several  other  changes  take 
place.  The  chromosomes,  without  otherwise  changing  their 
grouping  become  still  more  crowded  together,  and  thus  become 
massed  in  a  nearly  flat  plate,  while  the  centrioles  move  still  far- 
ther apart  (fig.  23).  Shortly  after  the  division  these  relations 
are  unchanged  save  that  the  centrioles  are  still  farther  separated 
and  lie  nearly  on  opposite  sides  of  the  chromosome-group.  The 
asters  are  still  present,  and  between  them  lies  a  rather  large, 
irregularly  spindle-shaped  area.  It  is  difficult  to  say  whether 
this  should  be  regarded  as  an  actual  spindle;  but  delicate  fibrillae 
often  may  be  seen  extending  into  it  from  the  poles. 

The  chromosome-group  lies  somewhat  excentrically  within  this 
area  in  the  form  of  an  irregular  flattened  plate.  In  side-view 
(fig.  30)  it  is  usually  impossible  to  distinguish  more  than  a  few  of 
the  chromosomes.  In  face  view  also,  the  crowding  is  often  so 
great  that  the  grouping  can  not  be  exactly  made  out.  Here  and 
there,  however,  it  is  evident  that  the  original  grouping  has  not 
been  lost,  and  occasionally  plates  are  to  be  found  in  which  every 
chromosome  may  be  clearly  seen  (figs.  31,  32).  Study  of  such 


STUDIES  ON  CHROMOSOMES  355 

cases  makes  it  certain  that  no  fusion  or  process  of  disintegration 
has  taken  place,  nor  is  any  evidence  of  a  nuclear  vacuole  to  be 
seen.  The  chromosomes  still  retain  the  same  form  as  in  the  pre- 
ceding anaphases,  the  X  F-bivalent  lying  near  the  center,  and 
still  very  clearly  distinguished  by  its  markedly  bipartite  form.  A 
very  characteristic  feature  of  this  stage  is  the  massing  of  mito- 
chondrial  granules  on  one  side  of  the  spindle-area  as  seen  in  side- 
view  (figs.  30,  31).  This  results  from  the  fact  that  during  the 
division  the  chondriosomes  are  mainly  massed  around  the  spindle 
and  do  not  extend  to  any  great  extent  into  the  polar  areas  (fig. 
22).  After  completion  of  the  division  therefore  the  chondri- 
somes  still  lie  mainly  on  the  side  of  the  chromosome-plate.  In  a 
general  way  this  relation  persists  throughout  the  interkinesis. 
In  the  condition  just  described  the  cells  remain  until  the  prophases 
of  the  second  division.  It  is  probable  that  the  interkinesis  is  of 
rather  brief  duration,  because  in  some  cysts  all  stages  may  be 
found  between  the  closing  anaphases  of  the  first  division  and  full 
metaphases  of  the  second.  Cysts  may,  however,  be  found  in 
which  practically  all  of  the  cells  are  in  the  condition  described; 
from  which  it  may  be  inferred  that  a  brief  pause  follows  the  com- 
pletion of  the  first  division. 

4.  The  second  spermatocyte-division 

The  prophases  of  the  second  division,  which  follow  directly 
upon  the  stage  just  described,  are  marked  by  a  resumption  of 
activity  on  the  part  of  the  astral  systems,  which  rapidly  increase 
in  development  while  a  definite  spindle  is  formed  between  them. 
As  this  takes  place,  the  chromosomes  spread  further  apart  and 
take  up  a  position  at  the  equator  of  the  spindle  in  the  same  group- 
ing as  before.  It  is  rather  difficult  to  follow  this  change  completely, 
as  these  stages  are  not  very  abundant  in  my  preparations,  and 
are  almost  always  seen  in  oblique  view.  It  is,  however,  certain 
that  a  double  movement  of  the  chromosomes  takes  place,  involv- 
ing (1)  a  rotation  of  each  chromosome  through  about  90  degrees, 
so  as  to  assume  a  position  with  its  long  axis  parallel  to  the  axis  of 
the  spindle,  and  (2)  a  virtual  rotation  of  the  entire  group,  so  as 


356  EDMUND  B.  WILSON 

to  lie  at  right  angles  to  the  spindle.  I  am  uncertain  whether  the 
latter  movement  is  a  simple  rotation  of  the  group  as  a  whole,  or 
whether  the  relative  position  of  the  individual  chromosomes 
changes  more  or  less  as  they  spread  apart.  It  is  certain,  how- 
ever, that  when  the  metaphase  has  been  attained  (figs.  34-37, 
photo.  4)  the  chromosomes  have  the  same  general  grouping  as  in 
the  final  anaphases  of  the  first  division  or  in  the  interkinesis,  save 
that  they  are  less  crowded.  As  before,  the  XF-bivalent  lies 
near  the  center,  surrounded  by  the  seven  other  chromosomes, 
very  often  arranged  in  an  irregular  ring,  though  this  is  somewhat 
variable. 

It  seems  probable  from  the  facts  just  described  that  in  these 
animals  the  general  grouping  of  the  chromosomes  is  determined  in 
the  prophases  of  the  first  spermatocyte-di vision.  Already  at 
this  time  the  X-  and  F-chromosomes  are  brought  into  position 
for  their  ensuing  conjugation;  and  their  topographical  relation 
to  the  autosomes  remains  thenceforward  unchanged  until  their 
final  delivery  to  the  spermatid-nuclei.  In  this  respect  these 
species  agree  with  such  forms  as  Fitchia  or  Rocconota  among  the 
reduvioids  (Payne,  '09)  and  differ  from  the  coreids  and  other 
forms  in  which  a  marked  change  of  grouping  occurs  after  the  first 
division.  I  conclude,  further,  that  neither  the  chromosomes  nor 
the  centrioles  lose  their  identity  in  the  period  between  the  first  and 
second  divisions,  and  that  a  complete  relation  of  continuity  exists 
between  the  two  generations  of  spermatocytes  in  this  respect. 

In  side-views  of  the  second  metaphase  the  .XT-bivalent  is  still 
almost  always  distinguishable  from  the  other  chromosomes  by 
its  deeply  constricted  dumb-bell  shape  (figs.  36,  37,  42,  43);  and 
in  correlation  with  this,  this  element  is  apparently  always  the 
first  to  divide,  its  two  components  having  often  completely  sepa- 
rated before  the  others  have  even  become  deeply  constricted 
(figs.  38  to  41,  44,  46,  photo.  6).  This  precocious  division  of  the 
XF-bivalent  is  a  very  common  phenomenon  among  the  Hemip- 
tera  (as  I  have  heretofore  described).  It  is  obviously  due  to  the 
comparatively  loose  union  of  X  and  F  after  their  conjugation,  so 
that  they  yield  more  readily  to  the  poleward  force  (whatever  it 
may  be)  that  operates  during  the  division. 


STUDIES  ON  CHROMOSOMES  357 

The  later  stages  of  this  division  have  been  so  often  described 
as  to  call  for  no  further  account.  The  final  result  is  that  the  sper- 
matid-nuclei  receive  the  haploid  number  of  chromosomes — seven 
in  Lygaeus,  eight  in  Oncopeltus — half  the  nuclei  in  each  case 
receiving  X  and  half  F.  The  facts  seen  so  clearly  in  both  these 
species  remove  every  possible  doubt  that  the  X-  and  Y-chromo- 
somes  which  thus  enter  the  spermatid-nuclei  are  the  same  individual 
chromosomes  that  conjugate  at  the  end  of  the  first  division  and  persist 
throughout  the  interkinesis  to  disjoin  in  the  course  of  the  second 
division.  It  is  of  course  possible  that  some  exchange  of  material 
may  take  place  between  them  during  the  brief  period  of  their 
association.  Of  this,  however,  there  is  no  evidence;  and  it  is 
certain  that  their  individual  boundaries  are  not  lost  to  view,  and 
that  not  even  an  apparent  fusion  takes  place  at  this  period  or 
any  earlier  one. 

5.  The  size-relations  of  the  sex-chromosomes  in  Oncopeltus 

In  my  first  examination  of  this  species  my  attention  was  given 
mainly  to  some  excellent  preparations  from  two  individual  males 
(designated  by  the  numbers  711  and  712)  in  which  the  X-  and  Y- 
chromosomes  appear  equal  in  a  large  majority  of  the  nuclei. 
The  facts  in  Nezara  hilaris  (Wilson,  '11  a)  led  me  to  extend  the 
examination  to  other  individuals  of  Oncopeltus,  when  to  my  sur- 
prise one  individual  was  found  (later  two  others)  in  which  a  slight 
but  evident  inequality  was  obvious  in  a  large  percentage  of  the 
cells  at  all  stages.  Upon  reexamination  of  the  entire  series 
the  interesting  discovery  was  made  that  in  every  individual  cases 
could  be  found  of  both  equality  and  inequality,  thb  ratio  between 
them  varying  widely  in  different  individuals.  In  the  extreme 
cases  this  is  perfectly  apparent  to  the  eye,  so  that  individuals  of 
predominantly  equal  or  unequal  type  may  readily  be  distinguished 
even  by  casual  inspection.  In  other  cases  one  is  often  in  doubt 
until  large  numbers  of  the  nuclei  have  been  tabulated.  As  exam- 
ples of  the  extreme  types  I  give  below  the  results  of  a  study  of 
two  individuals  (nos.  712  and  760)  representing  the  best  material 
as  to  fixation  and  staining.  In  these  a  comparison  of  the  X-  and 


358 


EDMUND  B.  WILSON 


F-chromosomes  as  to  apparent  size  was  made  in  one  hundred 
nuclei,  taken  at  random,  from  each  of  the  following  five  stages  of 
the  spermatogenesis :  (1)  the  pre-synaptic  leptotene,  (2)  the  synap- 
tic  period  (synizesis),  (3)  the  post-synaptic  spireme,  (4)  the  first 
spermatocyte-metaphase,  (5)  the  second  spermatocyte-metaphase. 
The  best  of  these  stages  for  the  purpose  are  the  maturation-divi- 
sions as  seen  in  side-view  (because  of  the  elimination  of  foreshort- 
ening) and  the  pre-synaptic  leptotene  (because  of  the  clearness 
with  which  both  sex-chromosomes  may  be  seen  at  this  time),  but 
in  neither  individual  was  the  requisite  number  of  side-views  of  the 
first  division  available.  In  the  latter  case,  therefore,  both  side- 
views  and  polar  views  have  been  included.  The  cases  are  classed 
as  equal  (eq.),  unequal  (uneq.)  and  doubtful  (dbf.),  the  latter 
including  those  in  which  there  was  reason  to  suspect  error  due  to 
foreshortening  or  the  like. 


PRE-SYNAPTIC 

8YNAPTIC 

POST-SYNAPTIC 

FIRST  DIVISION 

SECOND  DIVISION' 

eq. 

uneq. 

dbf. 

eq. 

uneq. 

dbf.' 

7 
4 

eq. 

uneq. 

dbf. 

eq. 

uneq. 

dbf. 

eq. 

uneq. 

dbf. 

712 
760 

83 
5 

10 
93 

7 
2 

80 

7 

13 

89 

65 

5 

23 
86 

12 
9 

55 
2 

37 

95 

8 
3 

83 
6 

9 

87 

8 

7 

There  is  no  doubt  a  considerable  error  in  these  figures  due  to 
foreshortening,  since  these  chromosomes,  though  often  spheroidal, 
are  often  slightly  elongated  (ellipsoidal),  and  are  of  course  seen 
in  all  positions.  But.  after  making  a  large  allowance  for  this,  the 
contrast  between  the  two  individuals  is  manifest  at  every  stage 
of  the  spermatogenesis,  and  nowhere  more  so  than  in  side-views 
of  the  second  division.  I  have  made  tabulations  of  several  other 
individuals  which  give  percentages  of  equality  ranging  from  ninety 
down  to  ten;  but  in  most  cases  the  data  from  intermediate  types 
are  less  consistent  and  the  probable  error  is  much  larger,  for 
obvious  reasons.6 

From  these  observations  I  draw  the  conclusion  that  in  Onco- 
peltus   the  X-  and    F-chromosomes  show  a  certain  tendency 

5  Compare  figs.  9, 10, 14, 15,  24,  25  (equal  type,  no.  711),  18,  19,3  6-40  (equal  type, 
no.  712)  with  11, 12,  16,  17,  20,  31,  32,  42  to  44  (unequal  type,  no.  760).  See  also 
photos.  7  to  11. 


STUDIES  ON  CHROMOSOMES  359 

towards  inequality  in  all  individuals,  so  marked  in  some  cases  as 
to  characterize  a  large  percentage  of  the  cells,  so  slight  in  others 
that  it  can  not  be  distinguished  by  the  eye  in  more  than  a  small 
percentage.  A  noteworthy  fact  remains  to  be  mentioned. 
Among  my  few  smear-preparations  of  Oncopeltus  is  one  slide, 
showing  great  numbers  of  nuclei  at  nearly  all  stages,  in  which  the 
.ST-chromosomes  are  almost  always  equal  in  the  growth-period 
and  earlier  stages  but  invariably  unequal  in  the  prophases.  I 
distrust  this  evidence  somewhat,  for  it  is  notorious  that  variations 
of  size  are  very  readily  produced  in  smears  owing  to  different 
degrees  of  flattening.  Were  this  the  only  explanation,  however, 
we  should  expect  to  see  the  size-differences  as  great  in  the  earlier 
as  in  the  later  stages.  If  the  result  be  trustworthy,  it  is  interest- 
ing as  indicating  the  existence  of  some  kind  of  material  differ- 
ence between  X  and  Y  that  is  expressed  in  a  greater  enlargement 
of  one  of  them  at  the  period  when  both  expand  somewhat  and 
undergo  longitudinal  splitting. 

II.  THE- GROWTH-PERIOD 

For  the  direct  study  of  the  actual  process  of  synapsis  and  its 
relation  to  the  reduction-division,  Oncopeltus  and  Lygaeus  pre- 
sent practical  difficulties  that  I  have  thus  far  found  insuperable; 
hence  no  attempt  will  be  made  to  describe  synapsis  in  detail. 
The  transformations  of  the  chromatin  during  the  growth-period 
will  nevertheless  be  considered  at  some  length,  partly  in  order 
to  trace  the  complete  history  of  the  sex-chromosomes,  partly 
because  of  the  interest  of  many  features  presented  by  the  auto- 
somes,  and  I  will  also  describe  certain  facts  observed  in  other 
animals  that  may  help  to  elucidate  some  of  the  problems  here 
encountered. 

1.  Outline  of  the  stages 

In  Oncopeltus  it  is  necessary  to  distinguish  not  less  than  twelve 
well  marked  stages  following  the  last  spermatogonial  division,  as 
follows : 

a.  (Figs.  47  to  49.)  The  final  spermatogonial  telophases,  in 
which  the  anaphase-chromosomes  break  up  into  a  confused  net- 


360  EDMUND  B.  WILSON 

like  structure,  and  for  a  short  time  their  boundaries  can  not  cer- 
tainly be  distinguished.  In  the  latter  part  of  this  stage  the  X- 
and  F-chromosomes  become  clearly  recognizable  as  compact, 
deeply  staining  bodies;  but  in  the  earlier  stages  they  too  seem  to 
be  in  a  diffused  condition.  This  stage,  of  short  duration,  corre- 
sponds to  Davis's  'Stage  a'  in  the  Orthoptera  ('08),  and  probably 
may  be  compared  to  the  'resting  stage'  that  has  been  described 
as  following  the  last  diploid  division  in  many  other  forms. 

b.  (Figs.  50  to  51.)     Post-spermatogonial  nuclei  of  somewhat 
larger  size,  in  which  the  chromatin  appears  in  the  form  of  separate, 
massive  bodies,  approximately  equal  in  number  to  the  chromo- 
somes of  the  diploid  groups.     Two  of  these,  of  more  even  contour 
and  staining  more  deeply,  are  now  recognizable  as  the  sex-chro- 
mosomes.    The  other  masses  are  more  or  le'ss  irregular  in  form, 
often  ragged  in   texture,  and  stain  more  lightly. 

c.  (Figs.  52  to  55.)     The  lightly  staining  masses  are  in  this 
stage   transformed  into   delicate,   closely   coiled   or   convoluted 
threads,  while  the  sex-chromosomes  retain  their  massive  form, 
and  are  thus  rendered  very  conspicuous.     In  the  latter  part  of 
this  stage  the  fine  threads  are  seen  uncoiling  or  unravelling  from 
the  massive  bodies  to  form  the  leptotene-threads  of  the  following 
stage. 

d.  (Figs.  56  to  59.)     The  pre-synaptic  leptotene.     The  auto- 
somes  now  have  the  form  of  long  delicate  threads,  while  the  sex- 
chromosomes  retain  their  massive  form  as  'chromosome-nucleoli.' 

e.  The  synaptic  stage  or  synizesis  (figs.  60  to  61).    The  threads 
are  now  much  thicker,  stain  more  deeply,  and  are  closely  convo- 
luted in  a  contraction-figure  or  synizesis.     A  plasmasome  can 
sometimes  be  distinguished  at  this  time,  but  is  usually  first  seen 
in  the  ensuing  stage. 

f.  Post-synaptic   spireme  (pachytene,   diplotene,  figs.  62  to 
65) .     Separate  thick  threads  are  now  again  spread  through  the 
nucleus,  approximately  of  the  haploid  number,  and  in  the  latter 
part  of  the  period  longitudinally  divided.     The  plasmasome  is 
now  nearly  always  present,  though  rather  small. 

g.  The  diffuse  or  confused  stage  (figs.  66  to  67).     The  double 
segmented  spireme  disappears  from  view,  giving  rise  to  a  rather 


STUDIES  ON  CHROMOSOMES  361 

coarse,  vague,  lightly  staining  net-like  structure,  in  which  are 
suspended  the  chromosome-nucleoli  and  the  plasmasome,  the 
latter  at  its  maximum  size.  In  this  stage  the  nuclei  remain 
throughout  the  greater  part  of  the  growth -period. 

h.  Early  prophases  (figs.  105,  107).  The  staining  capacity  of 
the  chromatin  increases,  while  more  definite  and  apparently  single 
threads  are  evident.  The  sex-chromosomes  are  more  elongate 
and  longitudinally  split.  The  plasmasome  now  diminishes  in 
size  and  disappears. 

i.  Middle  prophases  (figs.  108  to  114).  The  threads  rapidly 
condense,  stain  more  deeply,  and  draw  together  to  form  tetrad- 
rods,  double  crosses,  double  F's,  or  (rarely)  double  rings.  The 
sex-chromosomes  are  short  rods,  longitudinally  split. 

j.  Late  prophases.  In  these  all  the  chromosomes  are  converted 
into  compact,  deeply  staining  dumb-bell  shaped  bodies,  rarely 
quadripartite  in  outline,  which  are  ready  to  enter  the  spindle. 
This  stage  is  often  found  in  the  same  cysts  with  the  preceding,  all 
intermediate  gradations  being  readily  seen. 

k.  The  division-period,  including  the  two  spermatocyte-divi- 
sions. 

1.  Differentiation  of  the  spermatids.  Spermiogenesis  in  the 
narrower  sense. 

With  various  modifications  the  foregoing  stages  are  found  in 
many  Hemiptera,  among  the  best  of  which  for  study  of  the  early 
stages  are  the  pyrrhocorid  species  Largus  cinctus  and  L.  suc- 
cinctus.  Some  doubt  exists  in  regard  to  Stage  a;  and  it  is  possible 
that  in  some  forms  (of  which  Largus  may  be  an  example)  the 
spermatogonial  chromosomes  do  not  lose  their  identity  at  this 
time  but  give  rise  directly  to  the  massive  bodies  of  Stage  b.  In 
some  cases  (Largus,  Pyrrhocoris,  Alydus)  the  latter  part  of  Stage 
g  is  characterized  by  a  second  synizesis  or  contraction-figure,  in 
which  the  autosomes  are  again  closely  massed  together.  In  such 
cases  the  early  prophases  are  much  more  difficult  to  analyze. 

I  feel  confident  that  the  seriation  of  the  stages  is  correctly  deter- 
mined— indeed  the  only  possible  doubt  concerns  the  earliest  pre- 
synaptic  stages.  The  seriation  is  indicated  by  the  general  topog- 
raphy of  the  testis,  which  consists  of  very  definite  lobes  in  which 

THE   JOTJBNAL  OF  EXPERIMENTAL  ZO6LOOT,  VOL.   13,  NO.  3 


362  EDMUND  B.  WILSON 

the  cysts  develop  progressively  in  a  nearly  continuous  series  from 
one  end  to  the  other.  All  the  cells  in  each  cyst  are  nearly,  but 
often  not  quite,  in  the  same  stage.  While  the  order  of  succession 
is  not  demonstrated  so  accurately  as  in  some  objects  (e.g.,  in 
Batracoseps)  it  is  placed  practically  beyond  doubt  by  the  study 
of  transitional  conditions  in  cysts  where  slightly  earlier  and  later 
stages  occur  side  by  side. 

Throughout  the  whole  complicated  series  of  changes  in  the  auto- 
somes,  the  sex-chromosomes  are  at  once  recognizable  at  every 
stage  (save  the  very  first)  by  their  condensed  and  deep-staining 
character.  Lygaeus  differs  from  Oncopeltus  in  the  fact  that  the 
Jf -chromosome  always  retains  a  rod-like  form  and  is  longitudin- 
ally split  at  least  as  early  as  Stage  /.  In  Oncopeltus  both  sex- 
chromosomes  remain  in  the  form  of  rounded  and  apparently  un- 
divided chromosome-nucleoli  up  to  Stage  h,  when  they  too  assume 
the  form  of  short,  longitudinally  split  rods.  At  the  period  of 
synizesis  the  X-chromosome  in  Lygaeus  shortens  somewhat, 
but  at  no  time  does  it  assume  the  rounded  form  characteristic 
of  Oncopeltus  and  many  other  forms.  In  this  respect  Lygaeus 
bicrucis  differs  from  L.  turcicus,  where  the  JT-chromosome  has  the 
form  of  a  much  elongated  and  longitudinally  split  rod  in  the  early 
post-synaptic  stages,  but  later  contracts  to  a  spheroidal  form 
(Wilson, '05  b) .  These  species  of  Lygaeus  remove  every  doubt, 
could  such  longer  exist,  of  the  identity  of  the  chromatic  'nucleoli' 
of  the  growth-period  with  a  pair  of  chromosomes. 

2.  The  pre-synaptic  period.     Stages  a  to  d 

The  study  of  this  period  in  these  animals  is  of  much  interest  in 
relation  to  a  series  of  questions,  frequently  raised  in  late  years, 
that  are  of  the  utmost  importance  for  the  theory  of  synapsis. 
These  are:  (1)  Are  the  leptotene-threads  of  this  period  chromo- 
somes? (2)  Is  their  number  equal  to  that  of  the  spermatogonial 
chromosome-groups?  (3)  Can  they  be  traced  directly  as  individ- 
uals to  the  anaphase-chromosomes  of  the  last  spermatogonial 
division?  As  will  be  seen,  the  facts  in  the  Hemiptera,  in  the  dra- 
gon-fly Anax,  and  in  certain  Orthoptera  give  good  reason  to 


STUDIES  ON   CHROMOSOMES  363 

answer  the  first  two  of  these  questions  in  the  affirmative,  while 
the  third  remains  unanswered. 

Stage  b.  It  will  be  advantageous  to  consider  this  important 
stage  before  that  which  precedes  it,  as  there  are  doubts  concern- 
ing the  latter;  This  stage  and  the  following  one  are  characteristic 
of  many  Hemiptera  and  Orthoptera,  and  is  seen  also  in  the  dra- 
gon-fly; and  some  of  these  forms  are  much  better  adapted  for  its 
critical  study  than  are  Oncopeltus  and  Lygaeus.6 

In  the  latter  forms  numerous  cysts  in  Stage  b  are  seen  in  the 
region  between  the  spermatogonial  cysts  and  the  synaptic  zone, 
often  abutting  directly  upon  the  former.  For  this  reason  I  long 
supposed  this  stage  to  follow  immediately  upon  the  last  spermato- 
gonial division,  i.e.,  to  be  the  last  spermatogonial  telophaso.  Such 
indeed  is  possibly  the  case  in  Largus,  as  already  stated;  but  in 
some  other  forms  it  is  certainly  separated  from  the  telophase  by 
an  intervening  net-like  stage.  In  Oncopeltus  and  Lj^gaeus  Stage 
b  is  characterized  by  rather  small  spheroidal  nuclei  in  which  may 
be  very  distinctly  seen  a  group  of  separate,  more  or  less  irregular, 
massive  chromatic  bodies,  the  number  of  which  is  approximately, 
in  some  cases  exactly,  equal  to  the  diploid  number  of  chromosomes 
(figs.  50,  51,  71,  72).  In  preparations  but  slightly  extracted 
(after  haematoxylin  or  saffranin)  all  these  masses  stain  alike- 
deep  blue  or  red.  Upon  further  extraction  a  very  striking  con- 
trast appears  between  two  of  these  bodies  and  the  others,  the 
former  retaining  their  deep  color  and  having  a  fairly  even  contour, 
while  the  latter  become  pale  and  are  more  or  less  irregular  in  shape. 
As  will  be  shown,  the  two  dark  bodies  are  the  X-  and  F-chromo- 
somes,  which  may  be  traced  individually  through  all  the  succeed- 
ing stages  up  to  the  spermatocyte-divisions.  In  Oncopeltus 
they  are  spheroidal  or  ovoidal  in  shape  and  nearly  equal  in  size 
(figs.  50,  51).  In  Lygaeus  the  ^-chromosome  is  much  larger 
than  the  F,  and  always  has  the  form  of  a  more  or  less  elongate 
rod,  which  shows  a  good  deal  of  variation,  being  sometimes  quite 
straight,  sometimes  curved  in  various  ways  (figs.  71,  72).  In 

6  In  my  fourth  'Study'  ('09  a)  I  gave  a  brief  account  of  this  stage  in  Pyrrhocoris, 
illustrated  by  photographs,  describing  it  as  a  '  spermatogonial  post-phase, '  but  did 
not  endeavor  to  work  out  the  history  of  the  autosomes. 


364  EDMUND  B.  WILSON 

Largus  and  Pyrrhocoris  but  one  dark  body  is  seen;  and  this,  as 
I  earlier  showed  in  the  latter  case,  is  the  unpaired  X-chromosome. 

These  massive  bodies  strongly  suggest  those  to  which  Overton 
('05,  '09)  has  given  the  name  of  '  prochromosom.es'  in  the  case  of 
plant  cells.  Since  however  they  differ  from  the  latter  in  some 
important  respects  I  will  not  here  employ  this  term;  and  for  a 
similar  reason  will  not  designate  them  by  Strasburger's  term 
'gamosomes'  ('05),  though  they  undoubtedly  give  rise  to  the  chro- 
mosomes that  enter  synapsis. 

Even  a  casual  inspection  of  these  nuclei  is  enough  to  show 
that  the  number  of  chromatic  masses  is  not  far  from  the  sper- 
matogonial  number  of  chromosomes,  while  here  and  there  a 
nucleus  may  be  found  in  which  this  number  may  be  exactly 
counted.  The  enumeration  is  most  readily  made  in  the  case  of 
Largus  cinctus  where  the  spermatogonial  number  is  eleven. 
In  this  species,  which  has  eleven  spermatogonial  chromosomes 
(photo.  33),  nuclei  may  readily  be  found  in  which  ten  of  the  paler 
chromatic  masses  may  be  definitely  counted.  In  L.  succinctus 
their  number  is  often  seen  to  be  about  twelve  (the  spermatogonial 
number  being  thirteen).  In  like  manner,  the  number  of  the  pale 
masses  in  Lygaeus  is  sometimes  seen  undoubtedly  to  be  twelve, 
in  Oncopeltus  about  fourteen,  the  spermatogonial  numbers  being 
respectively  fourteen  and  sixteen,  though  in  neither  of  these 
species  can  the  number  be  exactly  determined  in  many  cases.  I 
do  not  hesitate  however  to  draw  the  conclusion  definitely  that  in 
these  animals  the  full  diploid  number  of  separate  chromatic  masses 
is  present  in  a  stage  that  shortly  follows  the  last  spermatogonial  divi- 
sion and  precedes  the  formation  of  the  leptotene-threads.  In  the 
dragon-fly,  Anax  junius,  there  is  a  closely  corresponding  stage, 
but  in  this  case  all  of  the  chromatic  masses  stain  nearly  alike,  and 
the  X-chromosome  can  often  not  be  certainly  distinguished  until 
a  little  later. 

The  stage  described  above  evidently  corresponds  to  one  in  the 
Orthoptera  (Davis's  'Stage  b'  in  Dissosteira,  Chortophaga  and 
other  grasshoppers)  and  is  clearly  shown  in  some  of  McClung's 
slides.  In  all  these  forms,  however,  the  chromatic  masses  stain 
more  deeply  than  in  the  Hemiptera,  are  of  elongate  form,  and  are 


STUDIES  ON  CHROMOSOMES  365 

more  or  less  definitely  polarized.  In  Anax  and  the  Hemiptera, 
on  the  other  hand,  they  are  of  more  less  or  rounded  or  irregular 
form,  and  show  no  definite  polarization.  This  is  corelated  with  a 
corresponding  difference  in  the  form  and  position  of  the  spermato- 
gonial  anaphase-chromosomes.  In  the  Orthoptera  the  latter  are 
in  general  rod-shaped,  with  their  long  axes  parallel  to  the  spindle- 
axis;  in  Anax  and  the  Hemiptera  they  are  much  shorter,  often 
rounded  in  form,  and  with  their  long  axes  (when  distinguishable) 
lying  at  right  angles  to  the  spindle-axis.  The  conditions  described 
above  are  occasionally  varied  by  the  appearance  of  one1  or  two 
deep-staining  bodies  in  addition  to  the  sex-chromosomes,  usually 
of  smaller  size  (cf .  the  photographs  of  Pyrrhocoris  in  my  fourth 
'Study').  Owing  to  their  inconstancy  I  am  uncertain  as  to  thejr 
nature. 

Whether  all  of  these  chromatic  masses  are  chromosomes  is  a 
question  that  probably  can  not  be  directly  or  certainly  deter- 
mined in  the  case  of  Oncopeltus  and  Lygaeus.  We  must  rely 
here  upon  indirect  evidence.  But  there  can  be  no  doubt  that  two 
of  them  are  chromosomes,  for  the  two  deeply  staining  bodies  of 
Lygaeus  and  Oncopeltus  may  be  traced  step  by  step,  with  no  break 
of  continuity,  into  the  two  chromatic  'nucleoli'  of  the  synizesis  and 
all  succeeding  stages,  and  thence  throughout  the  growth-period  into 
the  X-  and  Y -chromosomes  of  the  maturation-divisions.  Since  the 
paler  bodies  correspond  in  number  to  the  spermatogonial  num- 
ber of  autosomes,  and  since  they  undoubtedly  give  rise  to  the  lep- 
totene-threads  that  enter  the  synaptic  stage,  it  is  at  least  a  fair 
inference  that  they  too  are  chromosomes,  or  are  destined  to 
become  such. 

Stage  a.  As  stated  above,  I  long  supposed  the  stage  just  de- 
scribed to  follow  immediately  after  the  last  spermatogonial  divi- 
sion; but  it  now  seems  certain  that  in  Oncopeltus  and  Lygaeus, 
as  in  the  Orthoptera  (Davis,  op.  cit.)  it  is  preceded  by  one  which 
more  nearly  approaches  the  condition  of  a  'resting'  nucleus.  In 
this  stage  only  the  sex-chromosomes  can  be  clearly  identified, 
and  there  is  reason  to  conclude  that  in  a  still  earlier  telophase 
not  even  these  can  be  distinguished. 


366  EDMUND  B.  WILSON 

In  certain  cysts  that  obviously  precede  those  of  Stage  b  the 
nuclei  are  still  smaller,  the  sex-chromosomes  more  elongated,  while 
the  autosomes  form  a  lightly  staining,  vague  net-like  structure 
in  which  individual  chromosomes  can  not  be  distinguished.  This 
stage  evidently  corresponds  to  Davis's  'Stage  a'  in  the  Orthop- 
tera,  and  is  well  shown  in  McClung's  preparations.  A  similar 
stage  has  been  described  by  several  other  students  of  the  Orthop- 
tera,  especially  by  McClung. 

It  is  difficult  to  represent  these  nuclei  accurately  in  drawings; 
but  a  fairly  good  idea  of  them  may  be  obtained  from  figs.  68  to  70, 
which  are  from  careful  studies.  They  seem  to  contain  a  rather 
coarse  and  close  network,  with  thickened  and  irregular  nodes  of 
varying  size  and  number.  In  both  species  the  sex-chromosomes 
are  more  elongated  than  in  Stage  b,  and  in  Lygaeus  the  X-chro- 
mosome  often  assumes  an  almost  vermiform  shape,  as  is  shown 
in  the  figures.  That  these  nuclei  follow  almost  immediately 
upon  the  last  spermatogonial  telophase  is  proved  both  by  their 
small  size  and  by  the  transitional  stages  seen  in  the  same  nuclei. 
This  is  most  clearly  seen  in  Lygaeus,  where  the  elongate  X-chro- 
mosome  enables  us  to  identify  the  early  spermatocytes  with  cer- 
tainty (these  chromosomes  do  not  appear  as  condensed  bodies 
in  the  spermatogonial  nuclei).  In  the  cyst  from  which  figs.  68  to 
70  were  drawn  both  sex-chromosomes  are  perfectly  clear  in  many 
of  the  nuclei,  but  in  many  the  F-chromosome  can  not  be  found, 
and  in  a  considerable  number  of  nuclei,  which  seem  to  lie  entirely 
within  the  section,  not  a  trace  of  either  sex-chromosome  can  be  seen 
(fig.  68).  In  this  particular  cyst  no  spermatogonial  divisions  are 
seen ;  but  in  other  cysts  in  the  same  region  of  the  testis,  nuclei  of 
exactly  the  same  type  as  those  last  mentioned  (with  neither  sex- 
chromosome  in  evidence)  are  seen  together  with  the  spermatogonial 
anaphases.  That  the  latter  are  the  final  spermatogonial  divisions 
can  not  be  proved;  but  in  Lygaeus  the  evidence  seems  nearly 
decisive  that  there  is  a  short  period  following  the  last  division  in 
which  the  identity  of  all  the  chromosomes  is  lost  to  view.  I  believe 
this  to  be  true  also  in  Oncopeltus,  though  the  evidence  is  less  satis- 
factory. On  the  other  hand,  it  is  possible  that  in  Largus  the  final 
anaphase-chromosomes  give  rise  directly  to  the  massive  bodies  of 


STUDIES  ON  CHROMOSOMES  367 

Stage  b.  It  is  at  any  rate  certain  that  the  telophase-chromosomes 
in  this  form  retain  their  identity  much  longer  than  in  Lygaeus, 
as  is  shown  by  figs.  74  and  75,  which  are  connected  by  all  inter- 
mediate stages  with  anaphase-figures  in  the  same  cyst.  In  a 
recent  paper  on  Euschistus,  Montgomery  ('11)  describes  a  stage 
that  seems  to  correspond  to  my  Stage  b,  and  identifies  the  massive 
bodies  with  the  telophase-chromosomes. 7  I  must  confess,  how- 
ever, that  neither  this  account  nor  my  own  observations  on 
Euschistus  convinces  me  that  this  is  correct.  It  seems  to  me  that 
we  have  as  yet  no  safe  demonstration  in  any  animal  that  the  pre- 
synaptic  chromosomes  are  actually  the  same  individual  chromo- 
somes as  those  of  the  last  diploid  division. 

I  am  unable  to  state  in  exactly  what  way  the  massive  bodies  of 
Stage  b  arise,  for  there  is  no  way  of  demonstrating  the  seriation  at 
this  time,  and  the  change  is  probably  effected  rapidly.  Differ- 
ent cysts  of  Stage  b  vary  considerably,  the  massive  bodies  being 
more  irregular  and  less  sharply  defined  in  some;  but  I  have  not 
gained  any  clear  idea  of  the  succession. 

Stage  c.  We  may  now  consider  the  most  interesting  changes  that 
take  place  during  the  transition  to  the  leptotene  stage,  the  earlier 
of  which  may  in  some  cases  be  seen  in  the  same  cysts  with  the 
preceding  stage.  In  Oncopeltus  and  Lygaeus  the  minuteness  and 
delicacy  of  the  structures  are  such  that  I  was  long  in  doubt  as  to 
how  the  process  takes  place;  but  Largus,  Anax,  and  some  of  the 
grasshoppers  constitute  a  series  in  which  the  same  essential  phe- 
nomenon is  seen  on  a  successively  larger  scale,  and  which  leaves 
no  doubt  as  to  its  nature.  In  all  these  forms  the  process  involves 
the  resolution  of  the  paler  massive  bodies  into  closely  convoluted 
or  coiled  threads,  which  then  uncoil  or  unravel  to  form  the  leptotene- 
threads  of  the  succeeding  stage.  The  sex-chromosomes,  on  the  other 
hand,  fail  to  undergo  such  a  transformation,  and  retain  their  mas- 
sive form,  though  in  some  cases  (Largus)  there  is  some  evidence 
that  they  too  may  have  an  internal  thread-like  structure. 


7  Arnold  ('08)  gives  a  similar  account  of  a  corresponding  stage  in  Hydrophilus, 
and  describes  the  massive  bodies  as  conjugating  directly  two  by  two,  before 
giving  rise  to  spireme-threads. 


368  EDMUND  B.  WILSON 

A  process  of  this  type  was  long  since  describe.d  by  Janssens  ('01) 
in  both  the  spermatogonial  prophases  and  the  pre-synaptic  nuclei 
of  Triton  (figs.  27,  67),  where  it  somewhat  resembles  the  resolu- 
tion into  threads  of  the  'nucleoli'  of  the  germinal  vesicle  of  the 
same  animal,  as  earlier  described  by  Carnoy  and  Lebrun  ('98). 
A  process  more  or  less  similar  was  described  by  the  Schreiners  ('06, 
'08)  in  the  post-spermatogonial  (pre-synaptic)  stages  of  Tomop- 
teris,  by  Pinney  ('08)  in  the  sperma'togonial  prophases  of  Phryno- 
+.ettix,  and  especially  by  Davis  ('08)  and  more  recently  by  Brunelli 
('11)  in  the  pre-synaptic  stages  of  Chortophaga,  Tryxalis  and  other 
grasshoppers ;  Gregoire  describes  a  similar  process  in  plant-cells, 
first  in  the  somatic  cells  of  the  root-tip  in  Allium  ('06,  p.  330), 
later  in  the  pre-synaptic  sporocyte-nuclei  ('07,  p.  391).  The 
analogous  relations  discovered  by  Bonnevie  and  other  recent 
observers  are  referred  to  beyond. 

In  Oncopeltus  as  the  process  begins,  the  pale  chromatic  mass'es 
become  looser  in  texture  and  more  ragged  in  contour,  and  each  of 
them  gradually  assumes  the  appearance,  though  somewhat 
vaguely,  of  a  closely  convoluted  thread  (figs.  52,  53).  In  the 
stages  that  follow  (figs.  54,  55)  the  coiling  becomes  looser,  so  that 
contorted  or  spiral  threads  are  clearly  evident,  and  at  the  same 
time  the  massive  bodies  progressively  disappear  from  view.  These 
stages  unmistakably  show  the  nature  of  the  process  that  is  tak- 
ing place.  It  is  now  clear  that  each  of  the  original  compact  masses 
(excepting  the  sex-chromosomes)  has  resolved  itself  into  a  tightly 
convoluted  thread,  which  is  uncoiling  to  form  a  leptotene-thread. 
The  spiral  or  contorted  course  of  the  threads  is  still  very  evident 
when  the  massive  bodies  as  such  have  disappeared  from  view 
(fig.  55),  but  is  finally  lost  in  the  completed  leptotene-stage  (figs. 
56  to  59) .  In  Lygaeus  the  process  is  closely  similar  and  requires 
no  separate  description.  Figs.  71  and  72  show  two  nuclei  in 
Stage  b,  in  each  of  which  twelve  of  the  paler  masses  can  be 
counted  (not  all  shown  in  the  drawing),  while  the  X-  and  F-chro- 
mosomes  are  conspicuously  seen.  Whole  cysts  full  of  these  nuclei 
are  seen  in  nearly  all  of  my  sections.  Figs.  73  a  and  73  b  show 
two  early  leptotene-nuclei  of  this  species  after  the  unravelling  is 
completed. 


STUDIES  .ON  CHROMOSOMES  369 

These  stages  have  .been  described  in  Oncopeltus  mainly  be- 
cause of  the  importance  of  following  the  sex-chromosomes  at 
this  period;  but,  as  already  mentioned,  they  are  shown  more 
clearly  in  Largus,  spermatogonial  telophases  of  which  .are  shown 
in  figs.  74  and  75,  and  Stage  c  in  figs.  76  to  78.  Photos.  26  and 
27  show  nuclei  of  this  form  in  Stages  6  and  early  c,  the  character 
of  which  I  hope  will  appear  in  the  reproductions.  The  threads 
are  here  coarser  and  show  a  more  definitely  spiral  disposition, 
especially  evident  as  the  uncoiling  progresses.  This  is  clearly 
evident  in  many  nuclei  in  the  negative  from  which  photo.  27  is 
reproduced.  Though  these  nuclei  are  still  rather  small,  they 
afford  demonstrative  evidence  in  regard  to  the  main  fact.  I 
am  further  confident  that  the  threads  are  separate  and  undivided, 
and  that  but  one  thread  is  formed  from  each  mass;  but  the  latter 
conclusion  is  less  certain  than  the  former.  In  the  dragon-fly, 
Anax,  the  facts  are  similar,  and  in  some  respects  still  more  clearly 
shown.  Stage  b  is  shown  in  fig.  85  (the  massive  bodies  all  deeply 
stained) ;  and  in  fig.  86  (closely  similar  to  Janssen's  fig.  67  of  the 
spermatogonial  prophases  of  Triton)  are  shown  three  nuclei  lying 
side  by  side,  in  which  appear  three  successive  stages  of  the  unravel- 
ling. The  spiral  disposition  of  the  threads  in  this  form  is  some- 
times conspicuous,  and  may  be  clearly  seen  because  of  the  tend- 
ency of  the  chromatic  masses  to  assume  a  peripheral  position  in 
the  nucleus.  Not  infrequently  are  seen  nuclei  like  fig.  87  in 
which  a  striking  effect  is  given  by  the  uncoiling  threads.  In  such 
cases  it  is  very  evident  that  the  spirals  are  single,  and  the  evi- 
dence is  strong  that  one  thread  is  forming  from  each  massive 
body. 

These  stages  may  be  studied  to  still  greater  advantage  in  the 
grasshoppers,  where  they  have  been  accurately  described  and  fig- 
ured by  Davis  ('08).  This  observer  describes  the  post-spermato- 
gonial  nuclei  (Stage  6)  a,s  containing  a  series  of  elongated  massive 
bodies,  very  definitely  polarized,  approximately  equal  in  number 
to  the  spermatogonial  autosomes,  and  having  "  approximately 
the  same  orientation  that  the  autosomes  had  during  the  preceding 
telophases  of  the  last  spermatogonial  division"  (op.  cit.,  p.  38). 
In  figs.  43  and  44  he  represents  the  unravelling  of  a  single  thread 


370  EDMUND  B.  WILSON 

from  each  of  these  masses,  and  concludes  that  each  of  the  latter 
thus  " becomes  converted  into  a  single  chromatin- thread."  A 
study  of  McClung's  preparations,  particularly  of  Achurum,  leads 
me  to  a  confirmation  of  this  conclusion.  Stage  b  is  better  seen 
in  the  slides  of  Phrynotettix  than  in  Achurum;  but  as  the  latter 
shows  the  unravelling  stages  more  clearly  a  figure  of  Stage  b  from 
this  form  is  here  given  (fig.  88).  In  Achurum  the  unravelling 
process  is  quite  unmistakable  (figs.  89  to  92,  less  highly  magnified 
than  the  other  figures  of  the  plate;  see  also  photo.  28).  The 
threads  here  form  closely  convoluted  knots  (much  like  those 
figured  by  Janssens  in  the  spermatogonial  prophases  of  Triton), 
and  a  spiral  arrangement  is  seldom  seen.  In  Phrynotettix  and 
Mermiria  the  process  is  less  evident,  but  appears  to  be  of  the  same 
general  nature. 

Especially  in  Largus,  Anax  and  Achurum  the  definiteness  of 
the  pictures  and  the  succession  of  the  stages  seen  side  by  side  in 
the  same  cysts  or  in  adjacent  ones,  entirely  excludes,  I  think,  the 
possibility  that  they  are  a  merely  accidental  appearance  due  to 
vacuolization  of  the  massive  bodies,  corrosion-products  or  fixa- 
tion-artifacts. The  only  question  is  whether  the  thread  that 
unravels  from  each  massive  body  is  single,  double  or  longitudinally 
divided.  I  am  nearly  certain  that  the  threads  are  single  and  undi- 
vided. In  this  respect  my  conclusions  agree  with  those  of  Davis, 
and  differ  from  those  recently  announced  by  Brunelli  ('11)  in  the 
case  of  Tryxalis.  This  author  describes  a  similar  unravelling  proc- 
ess but  believes  the  threads  to  consist  of  two  separate  longitudinal 
halves  which  result  from  a  longitudinal  split  that  is  evident  already 
in  the  preceding  telophases,  and  which  separate  still  more  widely 
as  they  uncoil.  The  evidence  for  both  these  conclusions  seems  to 
me  very  incomplete,  as  none  of  the  unravelling  stages  are  shown, 
and  the  massive  bodies  of  Stage  b  are  assumed  to  arise  directly 
from  the  telophase  chromosomes  without  proof  of  this  impor- 
tant point.  This  assumption  may  be  correct,  but  it  seems  more 
probable  that  Stage  a  has  been  overlooked  by  this  observer. 

The  question  here  involved  is  so  important  that  I  have  en- 
deavored to  reach  a  more  certain  result  by  study  of  the  analogous 
processes  seen  in  the  spermatogonial  prophases.  The  first  of  these 


STUDIES  ON  CHROMOSOMES  371 

cases,  as  already  mentioned,  was  described  by  Janssens  in  Triton. 
The  chromatin-masses  ('blocs')  from  which  the  spireme-threads 
unravel  are  here  of  irregular  shape,  and  show  no  polarization,  but 
are  nevertheless  believed  to  be  directly  traceable  to  the  preceding 
telophase-chromosomes.  The  threads  are  already  in  evidence  in 
the  latter,  and  sometimes  show  an  irregularly  spiral  course 
(Janssens's  fig.  80)  but  in  the  later  stages  are  irregularly  convo- 
luted in  a  manner  very  similar,  as  far  as  can  be  judged  from  the 
figures,  to  that  seen  in  the  pre-synaptic  stages  of  the  grasshoppers. 
Janssens  emphasizes  his  belief  that  in  general  a  single  thread  is 
formed  from  each  'block,'  though  in  certain  cases  the  latter  are 
double  and  give  rise  to  two  threads.  An  essentially  similar  proc- 
ess is  described  for  the  pre-synaptic  stages.  "La  premiere  trans- 
formation qu'on  observe  dans  les  auxocytes  est  analogue  a  celle 
qui  annonce  le  commencement  de  la  division  dans  les  spermato- 
gonies  et  consiste  en  un  resolution  des  blocs  de 

nucleine"  ('01,  p.  68).  An  essentially  similar  phenomenon  in 
the  spermatogonial  prophases  is  brilliantly  demonstrated  in  two 
admirable  slides  of  Phyrnotettix  by  McClung,  one  stained  with 
iron  haematoxylin,  the  other  by  Flemming's  triple  method.  In 
this  form  the  'chromatin  blocks'  are  elongate  and  polarized  (fig. 
93,  photos.  29  to  31),  and  the  thread  later  forms  a  beautiful  and 
very  definite  spiral,  as  was  first  described  by  Pinney  (08).  This 
observer  describes  the  spiral  threads  as  formed  separately  within 
the  vesicles  or  sacculations  to  which  the  preceding  anaphase- 
chromosomes  give  rise  (as  first  made  known  by  Sutton  '00,  in 
Brachystola  and  confirmed  by  several  others  subsequently). 

As  far  as  can  be  judged  from  the  figures  and  brief  description  of 
Pinney,  the  threads  are  formed  as  rather  loose  and  open  spirals 
directly  out  of  a  fine  reticulum  within  each  chromosome-vesicle. 
The  rather  limited  material  at  my  disposition  shows  somewhat 
different  conditions,  though  confirming  the  main  point.  The  con- 
ditions in  McClung's  slides  differ  from  those  described  by  Pinney 
in  that  none  of  the  resting  nuclei  or  early  prophases  show  the 
nuclear  sacculations  so  distinctly,  nor  is  the  chromatin  so  diffuse- 
differences  which  may  well  be  due  to  the  fact  that  none  of  the 
earlier  generations  of  spermatogonia  are  shown.  In  these  nuclei 


372      •  EDMUND  B.  WILSON 

the  thread-formation  is  preceded  by  a  stage  in  which  the  chro- 
mosomes appear  in  the  form  of  deeply  staining,  elongated,  and 
more  or  less  definitely  polarized  bodies  (fig.  93,  photo.  29),  ragged 
in  contour  and  loose  in  texture,  but  showing  as  yet  no  definite 
coiled  thread.  This  condition  must  shortly  precede  the  thread- 
formation  because  the  latter  may  clearly  be  seen  in  other  nuclei 
in  the  same  cysts.  Whether  these  bodies  are  individually  derived 
from  the  anaphase-chromosomes  of  the  preceding  anaphase  can 
not  here  be  determined,  but  Miss  Pinney's  observations  make  it 
highly  probable  that  such  is  the  case.  In  any  case,  already  in 
the  early  prophases  each  of  these  masses  is  seen  to  be  resolving 
itself  into  a  closely  coiled  or  convoluted  thread,  similar  to  that 
seen  in  the  pre-synaptic  stages  but  disposed  in  more  definitely 
spiral  fashion  (figs.  94  to  96,  photos.  30  to  32).  In  some  cases 
there  are  indications  that  each  of  these  spirals  is  still  enclosed  in 
a  more  or  less  separate  nuclear  sacculation,  in  other  cases  this 
can  not  be  seen.  In  some  cysts  the  threads  are  seen  tightly 
coiled  within  the  massive  bodies  at  one  side  of  the  cyst,  while 
stages  of  uncoiling  are  seen  progressively  towards  the  opposite 
side.  In  slightly  later  stages  all  gradations  are  seen  in  the  uncoil- 
ing of  the  threads  to  form  separate  threads,  which  still  show  a 
distinct  spiral  course  even  after  they  have  begun  to  shorten  and 
thicken  (fig.  96,  photo.  32).  From  this  stage  it  is  easy  to  trace 
every  step  up  to  the  time  when  the  prophase-chromosomes  are 
about  to  enter  the  metaphase.  The  longitudinal  division  is  not 
evident  until  the  uncoiling  is  well  advanced,  and  the  two  halves 
remain  in  close  apposition  until  the  metaphase. 
The  points  that  I  would  here  emphasize  are : 

1.  The  extreme  clearness  with  which  the  spiral  threads  are 
seen,  which  removes  every  possible  doubt  as  to  what  is  taking 
place. 

2.  The  fact  that  the  threads  are  separate  from  the  time  of 
their  first  formation.     Apparently  there  can  be  no  question  here 
of  a  -continuous  spireme. 

3.  The  transitional  conditions  seen  in  the  same  cysts,  which 
prove  these  stages  to  be  prophases,  not  telophases.     In  this  re- 


STUDIES  ON  CHROMOSOMES  373 

spect  the  phenomenon  is  different  from  that  discovered  by  Bon- 
nevie  ('08,  '11)  in  Ascaris  and  Allium,  where  the  spiral  thread  is 
formed  in  the  telophase-chromosomes  and  uncoils  to  form  the 
thread-work  of  the  resting  nucleus. 

4.  The  certainty  that  the  spirals  are  single,  not  double,  i.e., 
do  not  consist  of  two  interlacing  spirals,  as  has  recently  been  de- 
scribed in  the  telophase-chromosomes  of  Tryxalis  by  Brunelli 
('10),  and  in  the  final  anaphase-chromosomes  of  Amphibia  by 
Schneider  ('11)  and  Dehorne  ('11). 

5.  The  strong  evidence  thus  afforded  that  only  one  thread 
arises  from  each  chromatin-mass. 

There  can  be  no  doubt  that  the  process  here  so  clearly  demon- 
strated is  of  the  same  general  nature  as  that  seen  in  the  pre-synap- 
tic  nuclei  of  these  animals  and  of  the.  Hemiptera.  I  therefore 
consider  it  at  least  probable  that  in  the  latter  case  also  a  single 
thread  is  formed  from  each  chromatin-mass  and  hence  that  the 
number  of  pre-synaptic  leptotene-threads  is  equal  to  the  diploid 
number  of  chromosomes. 

Resume  of  Stages  a  to  c.  The  close  parallel  that  exists  between 
the  pre-synaptic  stages  of  the  Hemiptera,  Odonatata  and  Orthop- 
tera  is  obvious.  In  all  these  forms  the  pre-synaptic  chromosomes 
first  appear  in  the  form  of  massive  'prochromosome'-like  bodies, 
approximately  of  the  diploid  number,  of  which  one  (X)  or  two 
(X  and  F)  are  already  recognizable  as  the  sex-chromosomes  by 
their  more  compact  structure,  regular  contour,  and  deep-staining 
quality.  Each  autosome  is  converted  into  a  tightly  coiled  or 
convoluted  thread  which  ultimately  unravels  to  form  a  lepto- 
tene-thread  of  the  stage  which  immediately  precedes  synapsis. 
This  process  is  clearly  analogous  to  that  seen  in  the  spermato- 
gonial  prophases,  and  in  each  case  the  evidence  is  that  a  single 
thread  arises  from  each  massive  body.  The  pre-synaptic  lepto- 
tene-threads are  thus  seen  to  be  of  the  same  nature,  and  probably 
of  the  same  number,  as  the  spermatogonial  prophase-threads, 
and  are  therefore  to  be  regarded  as  forming  a  diploid  group  of 
chromosomes.  The  sex-chromosomes,  on  the  other  hand,  per- 
sist in  the  massive  form  to  constitute  'chromosome-nucleoli,' 


374  EDMUND  B.  WILSON 

which  may  be  traced  into  and  through  the  growth-periods.8 
It  is  however  very  doubtful  whether  the  massive  bodies  of  Stage 
b  can  actually  be  traced  individually  back  to  the  anaphase-chro- 
mosomes  of  the  preceding  division,  though  this  may  be  possible  in 
some  forms. 

Stage  d.  The  leptotene-nuclei.  It  is  impossible  to  draw  any 
definite  line  of  demarkation  between  this  stage  and  the  preceding 
one,  since  they  are  connected  by  insensible  gradations.  An  excel- 
lent idea  of  these  stages  is  given  by  Miss  Hedge's  careful  drawings 
(figs.  56  to  59;  cf.  photos.  7,  8).  In  the  earlier  nuclei  the  threads 
still  have  a  more  or  less  spiral  or  wavy  course,  and  still  show  dis- 
tinct evidence  of  clumping  together  in  masses.  A  little  later  both 
these  appearances  are  lost,  and  the  threads  form  an  evenly  dif- 
fused, delicate  spireme,  always  separated  from  the  nuclear  wall 
by  a  considerable  clear  space.  Still  later  the  threads  become 
somewhat  thicker,  more  open  in  arrangement,  and  stain  a  little 
more  deeply  (figs.  58,  59,  73  a,  73  b,  78  to  80). 

These  nuclei  are  now  ready  to  enter  the  synaptic  or  synizesis 
stage,  which  immediately  follows.  They  show  essentially  the 
same  characters  in  Oncopeltus,  Lygaeus,  Largus,  and  many  other 
Hemiptera;  but  their  composition  is  difficult  to  analyze  precisely. 
It  is  certain  that  the  spireme  is  not  continuous  at  this  stage,  for 
free  ends  of  the  threads  are  readily  seen;  but  the  number  of  threads 
can  not  be  determined.  In  general  they  show  no  trace  of  polariza- 
tion, though  in  Lygaeus  traces  of  such  an  arrangement  are  some- 
times visible.  Whether  the  threads  branch  or  not  is  a  very  diffi- 
cult question.  At  first  sight  they  give  the  impression  that  they 
do  branch;  and  in  my  fourth  'Study'  (on  Pyrrhocoris)  I  described 
them  in  fact  as  forming  a  "  net-like  structure  in  which  traces  of  a 
spireme-like  arrangement  may  sometimes  be  seen."  The  more 
carefully  one  studies  these  nuclei,  however,  the  more  doubtful 
this  becomes.  Certainly  the  threads  may  often  be  followed 

8  The  mitotic  transformation  of  the  massive  bodies  is  not  however  diagnostic 
of  the  autosomes,  for  in  some  of  the  Orthoptera,  as  McClung  and  his  successors 
have  shown,  the  Jf-chromosome  is  also  converted  into  a  closely  convoluted  thread 
at  a  later  period.  I  have  some  reason  to  suspect  that  the  Jf-chromosome  of  Largus 
may  also  consist  of  a  very  tightly  convoluted  thread  in  the  earlier  stages ;  but  there 
is  never  any  sign  of  its  uncoiling,  and  in  the  later  stages  it  appears  homogeneous. 


STUDIES  ON  CHROMOSOMES  375 

individually  for  a  considerable  distance  without  branching;  and  it 
it  my  belief,  after  prolonged  study,  that  the  threads  do  not 
really  branch  or  form  a  network,  though  such  an  appearance 
is  often  given  by  fine  strands  of  'linin'  (perhaps  coagulated  nuclear 
sap)  connected  with  the  threads. 

A  point  to-  be  emphasized  is  that  these  threads  do  not  show  the 
least  sign  of  longitudinal  division,  and  in  this  respect  offer  a 
marked  contrast  to  the  longitudinally  double  threads  seen  in  the 
post-synaptic  stages. 

As  Stage  c  passes  into  Stage  d,  the  contrast  between  the  sex- 
chromosomes  and  the  others  becomes  still  more  pronounced.  In 
Oncopeltus  the  former  often  become  nearly  spheroidal  in  shape, 
and  stain  so  intensely  as  to  appear  exactly  like  chromatic  nucleoli. 
In  Lygaeus  they  stain  with  equal  intensity,  but  still  retain  more 
or  less  of  a  rod-like  form,  particularly  in  case  of  the  Jf-chromo- 
some.  In  Largus  (as  in  Pyrrhocoris)  the  unpaired  X-chromosome 
becomes  as  a  rule  spheroidal.  In  all  these  forms  the  sex-chromo- 
somes always  occupy  a  peripheral  position  with  respect  to  the  mass 
of  chromatin-threads,  sometimes  in  the  clear  space  outside  the 
latter,  more  often  embedded  in  its  peripheral  zone.  A  very 
striking  fact  (to  which  I  formerly  called  attention  in  case  of  Pyr- 
rhocoris) is  that  the  sex-chromosomes  in  these  stages  are  always 
separated  from  the  threads  by  a  vacuole-like  space.  This  is  most 
conspicuous  in  Largus  (figs.  78  to  80)  where  the  vacuole  is  unusu- 
ally large  and  clearly  defined;  but  it  also  appears  in  the  other 
forms  when  seen  in  the  right  position.  No  definite  wall  to  the 
vacuole  can  be  seen,  but  the  chromatin-threads  are  often  seen 
encircling  its  outer  limit,  as  if  lying  upon  a  definite  substratum. 
This  fact  is  interesting  as  indicating  that  the  sex-chromosomes 
really  lie  in  separate  compartments  or  chambers  of  the  nucleus, 
even  though  their  walls  can  not  be  seen.  Is  this,  conceivably, 
true  of  other  chromosomes,  and  may  this  possibly  be  the  basis  of 
the  genetic  continuity  of  chromosomes  in  general? 


376  EDMUND  B.  WILSON 

3.  Stage  e.     The  synaptic  period.     Synizesis 

We  now  approach  a  problem  that  I  have  thus  far  found  insolu- 
ble in  these  animals,  and  which  will  therefore  be  considered  very 
briefly.  This  involves  the  changes  by  which  the  leptotene-nuclei 
pass  into  the  pachytene  stage,  which  here  begins  with.the  contrac- 
tion-figure, or  synizesis.  This  stage  is  initiated  by  a  rapid  thick- 
ening of  the  threads,  accompanied  by  an  increase  in  staining 
capacity  and  a  further  contraction  of  the  mass  which  they  form. 
A  very  good  idea  of  this  stage  may  be  obtained  from  fig.  60,  which 
is  carefully  studied  in  every  detail.  As  this  figure  shows,  the 
synaptic  knot  distinctly  shows  two  kinds  of  threads,  thick  and 
thin,  closely  convoluted,  but  showing  no  definite  polarization  or 
other  visible  arrangement  in  loops.  The  results  shows  that  the 
process  of  synapsis  must  be  in  progress  at  this  time,  but  the  clos- 
est study  has  thus  far  failed  to  reveal  the  true  relation  of  the 
thick  threads  to  the  thin,  and  I  doubt  the  practicability  of  deter- 
mining precisely  what  is  taking  place.  In  these  Hemiptera, 
as  Digby  has  recently  remarked  of  Galtonia,  "  synapsis  faces  one 
as  an  impenetrable  wall"  ('10,  p.  739).  A  little  later  the  synaptic 
knot  undergoes  still  futher  contraction  (fig.  61)  and  is  till  more 
difficult  to  analyze;  but  in  favorable  cases  it  may  be  seen  to  con- 
sist of 'thick  threads,  closely  convoluted,  and  still  showing  no 
trace  of  polarization.  This  stage  evidently  corresponds  to  the 
early  pachytene  of  other  forms;  but  the  ' bouquet'  figure,  so  char- 
acteristic of  many  animals,  seems  to  be  entirely  wanting  here, 
and  I  have  found  no  indication  of  it  in  any  of  the  Hemiptera. 

Of  one  hundred  nuclei  of  this  stage  in  Oncopeltus,  taken  at 
random,  seventy-five  showed  X  and  Y  entirely  separate,  some- 
times on  opposite  sides  of  the  synaptic  knot,  while  in  twenty-five 
cases  they  lay  side  by  side,  just  in  contact.  Not  one  of  these 
nuclei  has  been  found  after  a  search  of  many  hundreds,  in  which 
these  chromosomes  were  fused,  or  even  flattened  together.  In 
Lygaeus,  on  the  other  hand,  there  is  a  stronger  tendency  for  these 
chromosomes  to  come  together  at  this  time,  one  hundred  nuclei 
showing  them  separate  in  forty-five  cases  and  in  contact  in  fifty- 
five.  In  the  latter  case  they  are  often  pressed  together  to  form 


STUDIES  ON  CHROMOSOMES  377 

an  unsymmetrical  dumb-bell  shaped  body  (photo.  12)  but  are 
never  fused  to  form  a  single  body.  In  thirty-six  of  the  foregoing 
fifty-five  cases  in  Lygaeus,  -X"  and  Y  were  attached  end  to  end 
(photos.  12,  13),  in  seventeen  side  by  side  (photo.  13) 

There  is  a  good  deal  of  variation  in  the  degree  of  contraction, 
even  in  cells  of  the  same  cyst;  and  this  may  be  due  in  part  tcHdiffer- 
ences  of  response  to  the  fixing  agent.  That  the  contractioMfigure 
can  not  be  regarded  as  an  artifact,  however,  is  proved  by  the 
fact  (which  I  briefly  described  in  my  fourth  'Study')  that  in  some 
Hemiptera  it  may  be  readily  seen  in  the  living  cells,  as  has  also 
been  shown  by  other  observers.  Gates  ('08)  has  suggested,  in 
case  of  certain  plants,  that  the  synizesis  is  not  produced  by  a 
contraction  of  the  chromatin-mass  but  by  enlargement  of  the 
nucleus  due  to  rapid  accumulation  of  liquid  about  the  chromatin. 
Such  a  view  can  hardly  apply  to  these  insects,  I  think,  though 
studies  of  the  living  material  would  give  a  more  trustworthy 
result  than  those  upon  sections. 

The  synaptic  knot  often  lies  excentrically  in  the  clear  space. 
Just  outside  it,  or  embedded  in  its  periphery  lie  the  sex-chromo- 
somes, still  surrounded  in  many  cases  by  the  vacuole,  though  this 
is  now  less  evident.  They  retain  the  same  appearance  as  in  the 
preceding  stage,  except  that  in  Lygaeus  they  are  somewhat  shorter 
than  before.  In  none  of  these  Hemiptera  does  either  sex-chromo- 
some elongate,  or  show  any  definite  relation  to  the  nuclear  pole 
at  this  stage.  In  this  respect  these  animals  differ  markedly  from 
some  of  the  Orthoptera,  where  the  X-chromosome  becomes  elon- 
gated and  takes  part  in  the  general  polarization  of  the  chromatin 
in  the  'bouquet'  stage.  There  is  no  evidence  of  a  giving  off  of 
material  from  this  chromosome  or  from  the  nucleus  at  this  time.9 

4.  Stages  f  and  g 

Stage  /.  The  post-synaptic  spireme.  Pachytene  and  diplotene. 
In  the  stage  im'mediately  following  synizesis  the  chromatin- 
threads  quickly  spread  apart  thiough  the  nuclear  cavity,  and  are 

9  Cf.  Moore  and  Robinson  ('04)  and  Morse  ('09)  on  the  cockroach,  Buchner  ('09) 
on  Gryllus. 

THE   JOURNAL   OF   EXPERIMENTAL  ZOOLOGY,   VOL.    13,   NO.   3 


378  EDMUND  B.  WILSON 

now  very  clearly  seen  to  be  separate,  constituting  a  segmented 
spireme.  All  the  threads  still  stain  deeply  and  are  very  much 
thicker  than  in  the  leptotene-stage;  hence  these  nuclei  may  be 
called  the  pachytene-nuclei.  In  the  earlier  part  of  this  stage  it 
is  uncertain  whether  the  threads  are  longitudinally  split  or  not; 
in  many  cases  the  closest  study  fails  to  reveal  such  a  split  in  sec- 
tions (figs.  62,  63),  though  in  smears  (fig.  65,  photo.  10)  the  split 
is  very  clearly  seen  in  nuclei  that  seem  to  belong  to  this  period. 
In  the  later  part  of  this  period  the  threads  become  still  thicker 
and  shorter  and  very  often  show  a  conspicuous  longitudinal  cleft. 
This  is  less  readily  seen  in  Oncopeltus  and  Lygaeus  (figs.  64,  83, 
84)  in  which,  indeed,  the  threads  sometimes  do  not  show  a  trace 
of  such  a  cleft  at  this  time  (which  I  attribute  to  defective  fixa- 
tion) .  In  Largus,  on  the  other  hand,  the  cleft  appears  in  the  most 
conspicuous  way,  especially  in  sections  fixed  with  Hermann's 
fluid  (figs.  81,  82),  where  the  threads  are  often  seen  to  consist  of 
double  rows  of  granules  often  showing  a  distinctly  paired  arrange- 
ment. 

In  Oncopeltus  and  Largus  the  sex-chromosomes  are  at  this  time 
hardly  changed,  still  having  the  form  of  undivided,  rounded  chro- 
mosome-nucleoli.  In  Lygaeus,  the  F-chromosome  is  still  of  this 
type,  but  the  X-chromosome  (usually  near  the  nuclear  membrane) 
is  now  very  clearly  split  lengthwise  (fig.  84),  in  which  condition  it 
persists  from  this  time  throughout  the  whole  growth-period.  The 
plasmasome  is  considerably  larger  than  before  although  not  yet 
at  its  maximum  size. 

The  number  of  chromosomes  (separate  chromatin-masses)  is 
now  obviously  approximately  half  that  of  the  diploid  groups.  In 
Oncopeltus  and  Lygaeus  this  can  be  determined  only  approxi- 
mately; but  it  is  certain  that  the  number  is  not  far  from  the 
reduced  or  haploid  number — that  is  to  say,  there  are  in  Oncopel- 
tus, in  addition  to  the  two  chromosome-nucleoli,  about  seven 
separate  diplotene-threads,  in  Lygaeus  about  six.  In  Largus 
cinctus  (where  the  spermatogonial  number  is  eleven)  nuclei  may 
readily  be  found  in  which  the  number  of  double  threads  may  be 
exactly  counted.  Such  nuclei  show,  in  addition  to  the  single 
chromosome-nucleolus,  five  double  threads  (figs.  81,  82)  of  which 


STUDIES  ON  CHROMOSOMES  379 

one  is  much  longer  than  any  of  the  others  and  evidently  corre- 
sponds to  the  large  pair  of  chromosomes  that  are  a  constant  feature 
of  the  diploid  groups  in  this  species  (photos.  33,  34).  From  these 
facts  it  is~clear  that  each  of  the  double  threads  is  a  bivalent,  which 
corresponds  to  a  pair  of  spermatogonial  chromosomes,  and  that 
synapsis  must  have  taken  place  during  the  period  of  contraction  or 
synizesis,  as  many  other  observers  have  concluded  in  both  ani- 
mals and  plants.10  In  what  manner  synapsis  takes  place,  and 
whether  the  longitudinal  halves  of  the  diplotene-threads  represent 
the  original  conjugants  in  side  by  side  union  are  questions  that 
here  present  very  great  practical  difficulties  to  direct  observation. 

Stage  g.  The  diffuse  or  confused  period.  The  diplotene-nuclei 
now  undergo  a  remarkable  transformation,  characteristic  of  many 
Hemiptera,  in  the  course  of  which  the  double  threads  as  such 
completely  disappear  from  view,  giving  rise  to  a  diffuse,  lightly 
staining  net-like  stage  in  which  the  boundaries  of  the  individual 
bivalents  are  indistinguishable  (figs.  66,  67,  97,  photos.  11,  14, 
16).  In  Oncopeltus  and  Lygaeus  I  have  found  it  impossible  to 
arrive  at  any  clear  notion  as  to  the  exact  nature  of  this  transfor- 
mation. In  Hermann  preparations  of  Largus  all  the  transitional 
stages  are  shown  with  great  apparent  clearness,  yet  even  here  it  is 
difficult  to  reach  a  certain  result.  This  question — one  of  the  most 
important  involved  in  the  maturation-process — will,  I  believe, 
repay  careful  study  in  smear-preparations,  which  I  hope  to  under- 
take hereafter  with  more  adequate  material  than  I  have  at 
present. 

As  the  process  begins,  the  threads  become  less  regular  and  at 
the  same  time  longer  and  thinner,  while  the  longitudinal  cleft  is 
still  more  evident.  A  little  later  the  two  halves  of  the  double 
threads  become  more  or  less  contorted,  more  granular  and  irregu- 
lar in  structure,  and  at  the  same  time  are  often  seen  to  be  separat- 
ing in  an  irregular  way  (figs.  101-103).  By  the  continuation  of 
this  change  the  double  threads  as  such  disappear  from  view, 
and  the  whole  nucleus  is  traversed  by  rather  thin,  irregular,  con- 

10  In  Syromastes  Gross  ('04)  believed  that  the  somatic  or  diploid  number  of 
double  threads  could  be  counted  in  the  post-synizesis  stages,  and  that  synapsis 
took  place  at  a  later  period. 


380  EDMUND  B.  WILSON 

torted,  more  or  less  interrupted  granular  threads,  which  often 
seem  to  branch  more  or  less.  These  nuclei  show  a  certain  resem- 

» 

blance  to  the  pre-synaptic  leptotene-nuclei  of  Stage  d]  but  both 
their  position  in  the  testis  and  their  structure  render  a  confusion 
between  these  stages  impossible.  When  the  process  is  completed 
the  threads  are  greatly  diminished  in  staining-capacity,  seem  to 
branch  more  freely,  and  in  Oncopeltus  and  Lygaeus  often  give 
almost  the  appearance  of  a  network  with  thickened  nodes  (figs. 
66,  67,  97).  In  Largus,  however,  the  threads  remain  more  in 
evidence,  and  the  nuclei  do  not  so  nearly  approach  the  'resting' 
condition  (fig.  104). 

At  the  height  of  this  stage  it  is,  I  believe,  quite  impossible  to 
distinguish  the  individual  chromosomes  (bivalents)  or  to  analyze 
exactly  the  composition  of  the  nuclei.  I  nevertheless  incline  to 
the  conclusion  that  the  autosomes  do  not  actually  lose  their 
identity  at  this  time.  The  phenomena  which  follow  in  Stage  h, 
especially  as  shown  in  Protenor,  give  considerable  reason  to  con- 
clude that  the  prophase-figures  are  already  formed  in  the  diffuse 
stage  but  are  lost  to  view  by  their  intricate  extension,  contortion 
and  interlacing.  In  Euschistus,  as  recently  described  by  Mont- 
gomery ('11),  the  confused  period  is  much  less  marked;  and  this 
observer  believes  that  the  bivalents  may  be  individually  recog- 
nized at  every  period.  In  Tomopteris  and  Batracoseps  the  con- 
fused period  is  entirely  omitted. 

In  the  condition  described  the  nuclei  remain  throughout  the 
greater  part  of  the  growth-period.  In  Oncopeltus  the  sex-chro- 
mosomes remain  always  spheroidal  or  ovoidal  (photos.  11,  16) 
and  apparently  undivided.  In  Lygaeus  both  sex-chromosomes 
(of  which  a  more  detailed  account  is  given  at  p.  384)  are  rod-like 
and  longitudinally  split  (photos.  13  to  15).  In  Largus  the  X-chro- 
mosome  is  spheroidal  but  often  shows  a  small  but  very  distinct 
central  cavity.  In  all  these  forms  the  plasmasome  is  conspicuous 
throughout,  and  attains  its  greatest  size  in  this  stage.  In  Onco- 
peltus and  Lygaeus  the  chromatin  undergoes  no  contraction  dur- 
ing this  period.  In  Largus,  on  the  other  hand  (as  in  Pyrrhocoris, 
Alydus  and  some  others)  the  latter  part  of  this  period  is  charac- 
terized by  a  very  marked  second  contraction-figure  or  synizesis, 


STUDIES  ON  CHROMOSOMES.  381 

forming  a  spheroidal  and  rather  dense  mass  separated  from  the 
nuclear  wall  by  a  considerable  clear  space  (cf.  Gross's  figures  of 
this  stage  in  Pyrrhocoris,  '07). 

5.  Stages  h  to  j.     The  prophases 

a.  The  bivalents.  At  the  end  of  the  diffuse  period  the  nuclei 
undergo  a  rapid  change  which  marks  the  appearance  of  the  defini- 
tive prophase-chromosomes.  This  is  accompanied  by  a  progres- 
sive condensation  and  increase  of  staining  capacity,  which  reaches 
a  climax  in  the  final  prophases,  and  by  the  disappearance  of  the 
plasmasome.  These  changes  may  be  studied  to  better  advantage 
in  smears  than  in  sections,  and  is  better  shown  in  my  material 
of  Protenor  than  in  the  other  forms.  As  seen  in  sections,  the 
initial  stage  (figs.  105,  106)  shows  the  nuclear  threads  more  dis- 
tinct, less  crowded  and  straighter,  often  giving  an  appearance 
somewhat  similar  to  the  beginning  of  Stage  g,  but  the  bivalents 
are  not  yet  defined.  In  smears  of  Protenor  (figs.  115  to  117)  it 
is  clearly  apparent  that  the  threads  are  separate,  single  (i.e.,  not 
longitudinally  split)  and  much  contorted.  A  little  later  the  threads 
are  seen  to  be  forming  themselves  into  the  characteristic  bivalent 
figures,  still  in  a  very  diffuse  and  irregular  form,  but  plainly  show- 
ing their  individual  boundaries,  and  in  some  cases  also  their  char- 
acteristic forms  (figs.  107,  108,  118,  119,  photo.  17).  In  Pro- 
tenor  the  m-chromosomes  are  first  clearly  seen  at  this  time  but 
are  much  less  definite  in  contour  than  in  the  following  stage.  As 
the  condensation  proceeds  the  bivalents  become  more  definite 
in  shape  and  can  be  more  readily  analyzed.  In  Stage  i  (figs. 
109-14,  photos.  18  to  23)  they  have  the  forms  which  have  been 
familiar  to  us  since  the  early  work  of  Paulmier  ('98,  '99)  on  the 
Hemiptera.  The  most  characteristic  of  these  is  (1)  the  double 
cross,  consisting  of  four  arms,  at  right  angles  to  each  other,  and 
longitudinally  split.  The  four  arms  may  be  equal  in  length. 
More  commonly  one  pair  is  shorter  than  the  other.  In  the  later 
stages  the  four  arms  typically  lie  in  the  same  plane.  In  earlier 
ones  they  are  often  curved ;  and  the  two  longer  arms  may  be  curved 
towards  each  other  until  they  nearly  meet  to  form  a  ring.  (2) 


382  .  EDMUND  B.  WILSON 

In  some  cases  the  two  arms  actually  meet,  uniting  to  form  a 
closed  ring,  of  the  type  first  made  known  by  Paulmier  ('98)  and 
often  observed  since,  both  in  insects  and  in  other  animals,  such  as 
Tomopteris,  or  the  grasshoppers;  but  this  type  is  much  rarer  in 
Oncopeltus  and  Lygaeus  than  in  some  other  species.  (3)  The 
third  type  is  that  of  the  tetrad-rod,  which  consists  of  a  straight  rod, 
which  shows  both  a  longitudinal  split  and  a  transverse  median 
suture.  These  forms  are  readily  deducible  from  the  double  cross 
by  reduction  and  final  disappearance  of  the  lateral  arms,  the  posi- 
tion of  which  is  now  indicated  by  the  transverse  suture.  As  will 
be  shown  hereafter  (especially  in  the  case  of  Protenor)  the  double 
crosses  undergo  in  their  later  stages  precisely  this  change;  but  the 
evidence  indicates  that  some  of  the  tetrad-rods  never  pass  through 
the  double  cross  stage.  (4)  The  fourth  type  is  the  double- F, 
best  described  as  a  F-shaped  figure  that  is  longitudinally  split  in 
the  plane  of  the  two  branches,  from  the  apex  of  the  F  towards  the 
free  ends,  accompanied  by  a  greater  or  less  degree  of  separation 
of  the  two  halves  thus  produced.  Figures  of  this  type  are  espe- 
cially common  in  the  earlier  stages  (fig.  108,  photos.  17, 18)  and  may 
be  recognized  soon  after  the  beginning  of  Stage  h  in  a  much  more 
elongate  form,  as  shown  in  fig.  107,  photos.  17  (from  a  smear- 
preparation).  In  the  final  prophases  (Stage  j)  all  the  bivalents 
finally  condense  to  form  dumb-bell  figures,  though  the  double 
crosses  (now  much  condensed,  and  often  more  or  less  opened  out 
in  a  ring  form)  may  sometimes  still  be  distinguished  in  the  early 
metaphases.  In  the  course  of  this  process  the  lateral  arms  of  the 
crosses  sooner  or  later  disappear,  and  a  cross  constriction  appears 
at  the  points  where  they  have  been.  These  conditions  will  be 
more  fully  considered  later  in  the  case  of  Protenor. 

Owing  to  the  uncertainty  regarding  synapsis  and  the  impossibil- 
ity of  tracing  the  bivalents  individually  through  the  confused 
period,  it  is  not  possible  to  offer  more  than  a  somewhat  conjectural 
interpretation  of  the  origin  and  relationships  one  to  another  of 
these  various  forms.  Paulmier,  McClung  and  other  earlier  stu- 
dents of  the  insects  assumed  the  primary  type  to  be  a  tetrad-rod, 
representing  two  univalent  chromosomes,  united  end  to  end,  and 
longitudinally  split.  From  this  type  the  double  cross  was  assumed 


STUDIES  ON  CHROMOSOMES  383 

to  arise  by  a  drawing  out  of  the  central  region  of  each  longitudinal 
half  from  the  synaptic  point  to  form  the  '  lateral  arms'  of  the  cross. 
The  ring-form  was  supposed  to  arise  by  a  secondary  bending 
around  of  the  two  principal  arms  until  the  free  ends  united;  the 
F-forms  by  a  sharp  flexure  of  the  bivalent  at  the  synaptic  point 
(cf.  Paulmier,  '98,  '99).  On  the  whole,  however,  it  seems  to  me 
that  the  evidence  points  more  strongly  to  the  opposite  interpre- 
tation, first  clearly  worked  out  by  the  Schreiners  ('06)  for  the 
closely  similar  figures  seen  in  Tomopteris.  According  to  this, 
the  original  condition  is  that  of  two  parallel  threads  or  rods,  in 
parasynaptic  association,  each  of  which  sooner  or  later  undergoes 
longitudinal  fission.  The  rings  are  described  as  arising  by  an 
opening  apart  of  the  two  rods  along  their  middle  portions  while 
remaining  attached  by  both  ends;  the  V's  by  an  opening  apart 
from  one  end,  while  remaining  attached  at  the  other;  and  from 
the  latter,  by  complete  opening  out  of  the  two  limbs  until  they 
are  in  a  straight  line,  arise  the  tetrad-rods.  From  the  latter  the 
crosses  are  readily  derived  by  drawing  out  of  the  lateral  arms  in  the 
manner  assumed  by  Paulmier. 

Though  all  this  is  somewhat  hypothetical  as  applied  to  these 
insects,  I  consider  it  the  more  probable  view  for  several  reasons. 
The  first  of  these  is  the  evidence  that  the  lateral  halves  of  the  dip- 
lotene-threads  begin  to  separate  already  at  the  end  of  Stage  / 
as  the  nuclei  are  passing  into  the  confused  stage,  and  the  correlated 
fact  that  in  the  initial  prophases  the  bivalents  are  seen  drawing 
together  out  of  more  or  less  widely  separated  single  threads.  A 
second  is  the  prevalence  of  the  double  F-figures  in  the  early  pro- 
phases,  and  their  gradual  disappearance  as  the  prophases  advance. 
It  is  evident  that  these  F-figures  are  opening  apart  in  these  stages, 
not  closing  up.  Elongated  V's  with  their  limbs  often  nearly 
parallel  (figs.  107,  108,  photo.  17)  are  commonly  seen  in  smear- 
preparations  of  these  stages,  and  in  slightly  later  ones  all  inter- 
mediate stages  connect  them  with  the  tetrad-rods  or  double 
crosses  (figs.  109  to  114,  photo.  18).  These  facts  are  quite  inde- 
pendent of  any  particular  conception  of  synapsis,  but  they  seem 
to  fit  best  with  the  view  that  the  original  type  is  a  longitudinally 
double  rod  following  parasynapsis,  as  maintained  by  the  Schrei- 


384  EDMUND  B.  WILSON 

ners.  This  view  receives  strong  support  from  Montgomery's 
recent  paper  on  Euschistus,  in  which  all  stages  of  such  opening 
out  are  shown,  and  in  which  the  process  of  parasynaptic  union 
is  described  in  detail.  It  may  be  pointed  out  that  the  accurate 
figures  of  Sutton  ('02)  from  smear-preparations  of  Brachystola, 
are  entirely  in  accordance  with  such  an  interpretation,  as  has 
been  also  indicated  by  Gregoire  ('10).  I  nevertheless  adopt  this 
conclusion  only  in  a  provisional  way,  as  it  is  still  based  to  large 
extent  upon  indirect  evidence  much  of  which  is  not  inconsistent 
with  the  earlier  and  opposite  conception  of  Paulmier. 

The  facts  that- have  been  described,  especially  as  seen  in  Pro- 
tenor,  point  very  definitely  to  the  conclusion  that  the  initial  stages 
of  the  formation  of  the  bivalents  are  passed  through  with  as  the 
nuclei  pass  into  the  confused  stage,  and  that  they  do  not  really 
lose  their  identity  in  this  stage  but  are  only  lost  to  view  by  their 
looseness  of  structure,  great  extension,  and  intricate  entangle- 
ment. This  interesting  question  will  repay  more  adequate 
study;  for  if  my  conclusion  be  correct  it  may  help  us  to  solve  the 
difficult  problem  of  the  disentanglement  of  the  leptotene-loops 
in  the  synaptic  process  of  such  forms  as  Tomopteris  and  Batra- 
coseps  (cf.  p.  407). 

b.  The  sex-chromosomes.  The  history  of  the  sex-chromosomes 
during  these  stages  is  very  easily  followed  throughout,  particu- 
larly in  smear-preparations,  and  affords  a  complete  demonstra- 
tion of  their  identity  with  the  chromatic  '  nucleoli'  of  the  growth- 
period.  In  Stage  h  these  chromosomes  almost  always  lie  close 
against  the  nuclear  wall,  and  in  Lygaeus  still  show  but  little  change, 
both  retaining  the  form  of  short  longitudinally  split  rods.  In 
Oncopeltus  they  show  a  marked  change,  being  now  more  or  less 
elongated  into  a  rod-like  form,  often  a  little  irregular  in  shape,  and 
now  for  the  first  time  plainly  longitudinally  split  (figs.  105,  106). 
In  Stage  i  they  are  regular,  short,  compact  longitudinally  divided 
rods,  essentially  like  those  of  Lygaeus  save  for  their  nearly  equal 
size.  This  may  be  studied  to  best  advantage  in  smear-prepara- 
tions, where  the  composition  of  the  chromosome-groups  may  be 
completely  analyzed.  In  such  preparations  the  total  number  of 
chromosomes  in  Oncopeltus  is  nine,  including  seven  bivalents 


STUDIES  ON  CHROMOSOMES  385 

and  the  two  univalent  sex-chromosomes.  The  later  may  at  once 
be  recognized  by  (1)  their  smaller  size,  (2)  more  compact  texture, 
and  (3)  simple,  rod-like  form  and  longitudinal  split  (figs.  108  to 
110,  photos.  20  to  23).  In  both  sections  and  smears  all  grada- 
tions are  seen  between  these  nuclei  and  those  of  the  growth-period 
which  remove  all  doubt  as  to  the  identity  of  these  chromosomes 
with  the  chromatic  'nucleoli'  of  the  latter.  On  the  other  hand, 
it  is  easy  to  trace  these  chromosomes  step  by  step  through  the 
later  prophases  into  the  two  small  chromosomes  of  the  first  divi- 
sion. In  these  stages  the  double  rods  are  seen  progressively 
shortening  until  they  assume  the  dumb-bell  shape  in  which  they 
enter  the  spindle  (figs.  112-114).  It  is  clear  from  the  transi- 
tional stages  that  the  transverse  constriction  of  the  dumb-bell 
corresponds  to  the  original  longitudinal  split  of  the  rod  before  its 
shortening,  while  the  long  axis  of  the  dumb-bell  represents  the 
original  transverse  axis  of  the  rod.  The  apparent  'transverse' 
division  of  the  dumb-bell  is  therefore  in  reality  a  longitudinal 
division. 

We  may  with  advantage  consider  at  this  point  some  very  inter- 
esting features  presented  by  the  X-chromosome  in  Lygaeus  bicru- 
cis11  especially  during  the  stages  preceding  the  prophases.  In  the 
earliest  stage  (a)  this  chromosome  is  an  elongate,  almost  vermi- 
form body,  which  appears  homogeneous  in  structure  (figs.  69, 
70).  In  Stages  b  to  d  (figs.  71  to  73)  it  is  shorter  and  thicker, 
and  still  usually  appears  homogeneous,  though  in  much  extracted 
preparations  of  Stage  c  it  may  appear  longitudinally  divided.  In 
Stage  e  (synizesis)  it  is  considerably  shorter  and  shows  no  sign  of 
division  (photo.  12).  In  Stage/  it  is  again  more  elongated  and 
unmistakably  split  lengthwise  (photos.  13  to  15),  and  in  this  con- 
dition persists  throughout  the  whole  growth-period,  gradually 
shortening  in  the  prophases  until  it  assumes  a  dumb-bell  shape, 
quite  as  in  Oncopeltus  (photo.  25). 

At  every  period  from  the  post-synaptic  spireme  onwards  many 
cases  may  be  found  in  which  the  double  rod  appears  nearly  or 
quite  homogeneous  (figs.  84,  98,  99,  100  a,  b:  photos.  14,  15);  but 

11  This  account  applies  only  to  this  species.     The  facts  in  L.  turcicus  are  very 
different,  as  already  mentioned  (see  Wilson,  '05  b,  '06). 


386  EDMUND  B.  WILSON 

in  many  other  cases  it  very  clearly  shows  a  double  series  of  vari- 
cosities  that  are  accurately  paired  in  the  two  longitudinal  halves, 
as  if  the  rod  originally  consisted  of  a  series  of  large  granules  or 
segments  that  afterwards  underwent  longitudinal  fission.  Some 
of  the  various  forms  that  appear  are  represented  in  fig.  100. 
Of  these  forms  one  is  far  more  freOluent  than  any  of  the  others — 
that  which  shows  three  pairs  of  segments  (100  d),  which  in  the 
best  cases  are  very  sharply  marked,  in  others  less  distinct  though 
evident,  in  others  barely  perceptible.  In  some  cases  (usually 
more  elongate  forms),  four  segments  are  apparent  (fig.  100,  c), 
but  no  case  has  been  seen  with  more  than  four.  In  a  few  cases, 
where  the  rod  seems  to  be  shorter,  less  than  three  segments  appear, 
and  an  almost  quadripartite  form  results  (fig.  100  e).  Some  of 
these  cases  are  obviously  due  to  a  sharp  curvature  of  the  rod,  so 
that  in  foreshortened  view  only  the  end  segments  are  seen;  but 
I  have  seen  a  few  cases  in  which  the  rod  seems  to  have  simply 
shortened  and  two  pairs  of  the  segments  seem  to  have  fused  to- 
gether. In  fig.  100/  the  rod  seems  to  show  six  segments  (the  only 
such  case  seen);  but  it  is  nearly  certain  that  this  represents  the 
X-  and  F-chromosomes  lying  end  to  end,  as  a  separate  F-chro- 
mosome  can  not  be  found  elsewhere  in  the  nucleus.  This  case, 
as  well  as  others  where  F  is  separate,  indicates  that  the  F-chro- 
mosome  also  may  consist  of  segments,  but  not  more  than  two 
such  have  been  seen  in  any  case.  Figs.  100,  g  and  h,  show  two 
isolated  F-chromosomes  of  the  homogeneous  type. 

It  seems  to  me  hardly  possible  that  this  striking  appearance  is 
an  accidental  artifact,  first  because  of  the  frequency  of  the  tri- 
segmental  type,  and  second  because  of  the  correspondence  of  the 
segments  of  the  two  halves  in  each  double  rod,  which  is  often 
rendered  more  striking  by.  a  decided  inequality  of  the  segments 
(well  shown  in  fig.  100/)  in  each  half.  All  such  cases  that  I  have 
seen  show  the  segments  accurately  paired.  For  these  reasons  I 
believe  the  segmented  structure  to  be  comparable  to  the  linear 
arrangement  of  '  chromatin-granules'  so  often  described  in  the 
spireme-threads  of  the  ordinary  chromosomes,  and  to  be  an  expres- 
sion of  some  kind  of  internal  structure  in  the  Jf-chromosomes. 
These  facts  may  be  added  to  the  evidence  reviewed  in  my  preced- 


STUDIES  ON  CHROMOSOMES  387 

ing  'Study7  ('11  a)  that  the  X-chromosome  (like  other  chromo- 
somes) is  a  compound  body.  They  help  us  to  understand  how  an 
X-chromosome  that  is  originally  single  may  break  up  into  two 
or  more  components  that  behave  as  separate  chromosomes  in  the 
diploid  groups  but  become  associated  in  a  coherent  group  ('X- 
element')  at  the  maturation-period  (Payne,  '09,  Wilson,  '11, 
Edwards,  '10),  and  provide  a  still  more  definite  basis  for  the  con- 
clusion that  this  chromosome  may  be  the  bearer  of  many  other 
factors  than  the  one  for  sex  (Wilson,  '11,  Morgan,  '11,  Gulick,  '11). 
The  bearing  of  this  on  sex-limited  heredity  is  obvious. 

It  is  a  very  important  fact  that  at  no  time  in  their  history  do  the 
individual  sex-chromosomes  in  these  Hemiptera  exhibit  a  cross- 
form  or  tetrad  structure  comparable  to  that  which  is  so  characteristic 
of  the  bivalents.  Such  a  tetrad  structure  only  appears  when  the 
two  sex-chromosomes  are  united  to  form  a  bivalent — as  is  seen 
for  instance  in  Brochymena  or  Nezara  (Wilson,  '05  b,  '11  a). 
The  only  apparent  exception  to  this  is  the  .XT-chromosome  in 
Lygaeus,  as  already  mentioned;  but  this  exception  is  evidently 
only  apparent.  The  essentially  bipartite  structure  of  these  chro- 
mosomes is  a  significant  fact  that  is  obviously  correlated  with 
their  univalent  nature,  and  with  their  approaching  single  divi- 
sion in  the  course  of  the  two  spermatocyte-divisions.  The  wider 
implications  of  this  will  be  considered  in  Part  III,  in  connection 
with  the  facts  seen  in  Protenor. 

6.  Comparative  considerations  regarding  the  maturation-period 

A  comparison  of  the  growth-period  in  these  Hemiptera  with  the 
conditions  seen  in  such  forms  as  Tomopteris  or  Batracoseps  shows 
some  striking,  though  I  think  secondary  points  of  difference. 

In  the  first  place,  the  formation  of  compact,  massive  bodies 
from  which  the  leptotene-threads  unravel  in  the  pre-synaptic 
period,  which  is  so  characteristic  of  these  insects,  seems  not  to 
take  place  in  Batracoseps  and  some  other  forms;  though,  as  will 
be  indicated  beyond,  the  Schreiners  have  found  indications  of  an 
analogous  process  in  Tomopteris. 

Secondly,  the  polarized  amphitene,  or  'bouquet-stage,'  that  is 
characteristic  of  Tomopteris,  Batracoseps  and  other  forms,  seems 


388  EDMUND  B.  WILSON 

to  be  entirely  wanting  in  these  insects,  where  in  its  place  we  find 
the  closely  convoluted  and  apparently  non-polarized  synaptic 
knot  or  synizesis.  The  controversy  as  to  whether  the  latter  is  an 
artifact,  due  to  the  coagulating  effect  of  the  reagents,  seems  to 
be  terminated  by  the  fact,  determined  by  Sargant  ('97),Overton 
('05),  Berghs  ('04),  Oettinger  ('09),  and  myself  ('09  a,  '09  b) 
that  the  synizesis  may  be  clearly  seen  in  the  fresh  (living?)  mate- 
rial immediately  after  gentle  teasing  apart  of  the  cells  in  a  nor- 
mal fluid  (Ringer's  solution)  in  which  the  spermatozoa  continue 
actively  to  swim.  Neither  at  this  stage  nor.  in  those  that  imme- 
diately precede  or  follow  is  there  the  least  sign  in  these  animals 
of  an  elongation  of  the  sex-chromosomes  or  of  a  giving  off  of 
nuclear  material  to  the  protoplasm. 

Third,  in  Tomopteris  and  Batracoseps  the  pachytene-loops 
formed  in  synapsis  persist  as  such  throughout  a  large  part  of  the 
growth-period,  without  undergoing  at  any  period  an  apparent 
loss  of  identity  in  a  'diffuse'  stage  such  as  is  so  characteristic  of  the 
Hemiptera.  In  Batracoseps  the  pachytene-loops  become  longi- 
tudinally divided  ('diplotene')  near  the  end  of  the  growth-period, 
when  they  give  rise  directly  to  the  prophase-figures.  In  Tomop- 
teris the  diplotene-threads  are  apparent  at  a  much  earlier  period 
(Schreiner),  but  here  too  give  rise  directly  to  the  prophase-figures. 
In  the  Hemiptera  here  considered  the  diplotene  is  likewise 
formed  very  early,  but  the  diffuse  stage  is  interpolated  between  it 
and  the  definitive  formation  of  the  prophase-figures,  and  the 
greater  part  of  the  growth-period  is  passed  in  this  condition  (in 
some  cases  accompanied  by  a  second  contraction-figure  in  the 
later  period).  There  is,  however,  an  analogy  in  this  respect 
between  these  Hemiptera  and  Tomopteris,  where  the  Schreiners 
describe  and  figure  ('06,  p.  19,  figs.  31,  32)  a  stage  following  the 
early  diplotene  in  which  the  parallel  halves  of  the  double  threads 
become  longer,  thinner,  less  regular,  and  spread  more  or  less  widely 
apart,  though  still  retaining  their  connection  at  certain  points. 
It  is  very  probable  that  this  process  corresponds  to  that  which 
marks  the  beginning  of  the  diffuse  stage  in  the  Hemiptera,  but 
does  not  proceed  so  far;  and  that  in  this  respect  Tomopteris  is 
intermediate  between  these  animals  and  the  Amphibia.  Perhaps 


STUDIES  ON  CHROMOSOMES  389 

we  may  here  find  a  clue  to  the  more  extreme  forms  of  diffusion 
observed  in  the  oogenesis  of  many  animals  and  in  the  ordinary 
somatic  nuclei.12 

As  regards  the  problem  of  synapsis  and  reduction,  the  existence 
of  the  synizesis  and  diffuse  stages  renders  the  Hemiptera  very 
unfavorable  objects  as  compared  with  Tomopteris  or  Batraco- 
seps,  and  we  are  here  thrown  back  upon  analogies.  Emphasis 
may  however  be  laid  upon  the  essential  similarity  of  the  prophase- 
figures  in  Tomopteris  and  these  insects;  and  if  my  interpretation 
of  the  diffuse  stage  be  correct,  it  is  probable  that  these  figures 
have  essentially  the  same  mode  of  origin  from  the  diplotene- 
threads.  Following  this  analogy,  I  provisionally  assume  the 
latter  to  follow  an  original  side  by  side  union,  or  parasynapsis— 
not  an  end  to  end  union  or  telosynapsis,  as  was  assumed  byPaul- 
mier,  Montgomery  and  (in  Orthoptera)  McClung,  Button,  and 
more  recently  by  Davis.  In  his  latest  paper  ('11)  Montgomery 
rejects  his  former  interpretation  in  favor  of  the  one  here  adopted. 
If  the  double  cross-figures  (or  the  tetrad-rods)  arise  in  the 
manner  assumed,  it  is  clear  that  their  'transverse'  division  is 
the  last  remnant  of  the  original  longitudinal  cleft  of  the  diplo- 
tene-thread;  and  it  is  certain,  as  Paulmier  first"  showed,  that  this 
'transverse'  division  corresponds  to  the  planeof  the  first  sperma- 
tocyte-division.  If  we  accept  this,  and  if  the  original  longitudi- 
nal cleft  of  the  diplotene  corresponds  to  the  plane  of  synapsis, 
it  follows  that  the  first  spermatocyte-division  is  the  'reduction- 
division,'  as  Paulmier  and  Montgomery  concluded.  I  repeat, 
however,  that  this  conclusion  is  here  adopted  only  in  a  tentative 
way;  since  the  case  is  by  no  means  proved;  and,  as  will  appear, 
my  conception  of  the  reduction-division  differs  materially  from 
the  one  more  commonly  held. 

As  regards  the  sex-chromosomes,  on  the  other  hand,  all  is  clear. 
The  observations  here  recorded  remove  every  doubt,  I  think,  in 

12  A  more  or  less  wide  divergence  of  the  longitudinal  halves  of  the  diplotene- 
threads  appears  to  be  the  rule  among  many  animals  and  plants.  It  has  been  espe- 
cially emphasized  by  Gre"goire  ('04,  '10)  who  has  called  attention  to  the  striking 
contrast  in  this  respect  between  the  bivalent  chromosomes  of  the  maturation- 
period  and  the  longitudinally  split  spireme-threads  of  the  somatic  divisions.  See 
also  Strasburger,  '09,  pp.  98  to  100. 


390  EDMUND  B.  WILSON 

regard  to  the  following  points.  First,  it  is  certain  that  each  of  these 
chromosomes  divides  but  once  in  the  course  of  the  maturation-process, 
namely,  in  the  first  division;  and  this  division  is  clearly  longitudinal 
and  equational.  The  second  'division'  of  the  XY-pair  is  obviously 
not  a  division  at  all  but  only  the  disjunction  of  two  separate  chromo- 
somes that  have  for  a  short  time  been  in  contact  without  loss  of  their 
identity.  This  process  is  an  evident  and  typical  reduction-divi- 
sion in  the  original  sense.  In  these  animals,  therefore,  it  is  quite 
certain  that  the  XT-pair  undergoes  a  process  of  'post-reduction' 
(cf.  Wilson,  '05  c).  It  is  a  remarkable  fact,  proved  by  the  studies 
of  Stevens,  that  in  the  Coleoptera  and  Diptera  the  -XT-pair  fol- 
lows the  reverse  order,  as  is  also  the  case  with  the  w-chromosomes 
of  the  coreid  Hemiptera.13 

7.  Comment  on  the  sex-chromosomes  in  Oncopeltus 

The  extremely  close  correspondence  between  Oncopeltus  and 
Lygaeus  at  every  stage  of  the  spermatogenesis  leaves  not  the 
least  doubt  of  the  identity  of  the  sex-chromosomes  of  the  two 
forms.  Apart  from  the  size-differences  of  these  chromosomes, 
Lygaeus  differs  from  Oncopeltus  only  in  (1)  the  retention  through- 
out of  a  rod-like  form  by  the  JT-chromosome,  (2)  the  earlier  appear- 
ance of  the  longitudinal  split  in  both  sex-chromosomes,  (3)  a 
slightly  more  marked  tendency  for  the  sex-chromosomes  to  con- 
jugate at  the  time  of  general  synapsis.  On  the  other  hand,  the 
sex-chromosomes  of  the  two  forms  agree  in  all  the  characteristic 
peculiarities  of  these  chromosomes  shown  in  the  Hemiptera  gener- 
ally, namely,  (1)  the  retention  of  a  compact  and  deeply  staining 
character  from  an  early  pre-synaptic  period  down  to  the  spermato- 
cyte-prophases,  (2)  their  division  as  separate  univalents  in  the 
first  spermatocyte-di vision,  (3)  their  subsequent  conjugation  to 
form  a  bivalent,  which  occupies  a  nearly  central  position  in  the 
second  spermatocyte  metaphase-group,  and  (usually)  divides  in 
advance  of  the  other  chromosomes.  These  facts -fully  establish 

13 1  may  point  out  that  it  is  inadmissible  to  designate  as  'm-chromosomes'  any 
pair  of  especially  small  chromosomes  without  respect  to  their  other  characteristics, 
as  has  been  done  by  several  writers.  The  m-chromosomes  of  the  Coreidae  are  not 
always  distinctly  smaller  than  the  other  chromosomes,  and  they  are  characterized 
by  certain  very  definite  peculiarities  of  behavior.  Cf.  Wilson,  '05  c,  '11  a. 


STUDIES  ON  CHROMOSOMES  391 

the  identity  of  the  sex-chromosomes  in  Oncopeltus.  It  may  there- 
fore be  taken  as  an  established  fact  that  a  pair  of  sex-chromosomes 
may  be  recognizable  as  such  even  in  cases  where  they  show  no 
perceptible  difference  of  size,  and  where  no  constant  differences 
between  the  diploid  chromosome-groups  of  the  two  sexes  can  be 
seen. 

Such  cases  are  of  course  fatal  to  the  view  that  the  nuclear  differ- 
ences between  the  sexes  are  reducible  to  one  of  general  chromatin- 
mass;  and,  as  I  have  elsewhere  urged,  these  chromosomes  can 
be  regarded  as  factors  in  sex-production  only  by  assuming  some 
kind  of  difference  between  the  substance  of  X  and  Y.  I  will 
not  here  enter  upon  the  discussion  of  a  point  that  has  been  fully 
considered  in  several  earlier  papers  (see  especially  Wilson,'  11  a, 
'11  b).  I  will  only  again  express  the  view  that  the  differential 
factor  between  X  and  F  may  plausibly  be  regarded  a  specific 
chemical  substance  (the  '.X-chromatin')  that  is  either  confined  to 
the  ^T-chromosome  or  is  there  present  in  relative  excess,  and  in 
respect  to  which  the  two  sexes  differ  correspondingly.  If  this  is 
correct,  the  sexual  differences  may  be  at  bottom  dependent  upon 
a  fundamental  quantitative  difference  of  metabolism,  as  stated 
in  my  first  paper  on  this  subject  ('05  a).  Such  nuclear  differences 
between  the  sexes  may  of  course  exist  not  only  in  forms  where  no 
difference  of  to'tal  chromatin  mass  is  visible,  but  even  where  no 
special  'sex-chromosomes'  are  differentiated.  The  surprising 
thing,  indeed,  is  that  they  should  in  some  instances  be  expressed 
in,  or  accompanied  by,  visible  sexual  differences  of  the  chromo- 
some-groups. 

,   t  . 

III.  CRITICAL  CONSIDERATIONS  ON    THE  MATURATION-PHENOM- 
ENA BASED  ON  A  COMPARISON  OF  THE  HEMIPTERA, 
TOMOPTERIS,  BATRACOSEPS   AND  SOME 
OTHER  FORMS 

As  has  been  indicated,  my  conclusions  concerning  synapsis 
and  reduction  in  Hemiptera  are  largely  tentative  in  character.  If 
I  nevertheless  venture  to  make  some  critical  comment  on  the 
general  problem  it  is  mainly  because  of  the  opportunity  I  have  had 
to  reexamine  these  phenomena  in  Tomopteris  and  Batracoseps. 


392  EDMUND  B.  WILSON 

1 .     The  question  of  synapsis 

The  cytological  problem  of  synapsis  and  reduction  involves 
four  principal  questions,  as  follows:  (1)  Is  synapsis  a  fact?  Do  the 
chromatin-elements  actually  conjugate  or  otherwise  become  asso- 
ciated two  by  two?  (2)  Admitting  the  fact  of  synapsis,  are  the 
conjugating  elements  chromosomes,  and  are  they  individually 
identical  with  those  of  the  last  diploid  or  pre-meiotic  division? 
(3)  Do  they  conjugate  side  by  side  (parasynapsis,  parasyndesis), 
end  to  end  (telosynapsis,  metasyndesis)  or  in  both  ways?  (4) 
Does  synapsis  lead  to  partial  or  complete  fusion  of  the  conjugating 
elements  to  form  'zygosomes'  or  'mixochromosomes,'  or  are  they 
subsequently  disjoined  by  a  'reduction-division?'  Upon  these 
questions  depends  our  answer  to  a  fifth  and  still  more  important 
question,  namely,  (5)  Can  the  Mendelian  segregation  of  unit- 
factors  be  explained  by  the  phenomena  of  synapsis  and  reduction? 
Despite  the  prodigious  accumulation  of  data  regarding  these 
questions  the  unprejudiced  student  of  the  literature  finds  himself 
compelled  to  admit  that  not  one  of  them  has  yet  received  a  really 
demonstrative  answer — at  least  not  one  that  has  brought  convic- 
tion to  the  minds  of  all  competent  cytologists.  I  do  not  propose 
to  consider  them  exhaustively,  or  to  give  any  approach  to  a  com- 
plete review  of  the  literature.  This  has  been  done  by  other  writ- 
ers, notably  by  Gregoire  ('05,  '10)  in  two  extended  and  masterly 
memoirs,  by  Strasburger  in  a  most  valuable  series  of  critical 
essays  ('07,  '08,  '09,  '10),  and  by  Haecker  ('07,  '10).  (See  also 
Davis,  '08,  Gerard,  '09,  Gates,  '11,  Montgomery,  '11,  and  the 
series  of  papers  by  the  Schreiners  and  by  Janssens.)  I  will 
however  indicate  some  of  the  conclusions  to  which  I  have  been 
led  in  an  effort  to  form  an  independent  judgment  concerning  the' 
facts,  especially  in  Tomopteris  and  Batracoseps,  which  are  prob- 
ably unsurpassed  as  objects  of  observation,  have  become  classi- 
cal through  the  well  known  studies  of  the  Schreiners  ('06,  '08) 
and  of  Janssens  ('03,  '05),  and  have  formed  a  main  center  of  con- 
troversy in  recent  years. 

'  The  conclusions  of  these  observers  (more  especially  those  of  the 
Schreiners)  have  been  the  object  of  repeated  criticism  on  the  part 
of  Goldschmidt  ('06,  '08),  Fick  ('07,  '08),  Meves  ('07,  '08,  '11), 
Haecker  ('07,  '10)  and  many  others.  These  criticisms,  too  well 


STUDIES  ON  CHROMOSOMES  393 

known  to  call  for  extended  review,  were  substantially  at  one  in  the 
contention  that  what  had  been  described  as  a  parallel  or  sidewise 
conjugation  of  spireme-threads  during  the  'bouquet,'  'synap- 
tene'  or  'amphitene'  (synaptic)  stage  is  nothing  other  than  a 
modified  form  of  longitudinal  splitting,  in  which  double  threads, 
longitudinally  divided  from  the  beginning,  are  progressively  differ- 
entiated out  of  the  nuclear  substance  from  one  pole  of  the  nucleus 
towards  the  opposite  pole.  In  the  course  of  his  able  critique 
Meves  ('07)  endeavors  to  break  down  the  distinction  between 
such  a  process  and  that  which  is  seen  in  the  prophases  of  somatic 
cells,  contending  that  in  both  cases  the  longitudinal  duality  is 
brought  about  by  a  biserial  grouping  of  the  chromatin-granules 
of  the  resting  nucleus,  and  urging  that  the  process  seen  in  the 
amphitene-nuclei  is  of  essentially  the  same  nature  as  the  early 
division  of  spireme-threads  in  the  diploid  nuclei  long  ago  described 
by  Flemming.  Unquestionably,  this  objection  is  worthy  of  the 
most  attentive  consideration,  especially  in  view  of  the  conclusion 
of  several  recent  observers  (considered  more  in  detail  beyond) 
that  the  longitudinal  division  of  the  spireme-threads  is  in  some 
cases  already  in  evidence  in  the  chromosomes  of  the  preceding 
anaphases  or  telophases,  and  that  the  two  halves  thus  arising 
may  separate  more  or  less  widely  before  the  nuclei  have  entered 
the  'resting'  state.  For  Meves  there  is  no  problem  of  synapsis. 
The  Gordian  knot  is  cut  with  the  statement,  "Die  Geschlechts- 
zellen  bezw.  ihre  Kerne  haben  nach  meiner  Vorstellung  (1907) 
die  besondere  Eigenschaft  ererbt,  beim  Eintritt  in  die  Wachstums- 
periode  nur  die  halbe  Zahl  von  Chromosomen  auszubilden" 
('11,  p.  296).  Certainly  the  adoption  of  this  simple  solution 
would  save  a  great  deal  of  trouble;  but  I  fear  that  the  facts  com- 
pel us  to  take  a  more  roundabout  way  out  of  our  difficulties. 
Goldschmidt  and  Haecker,  on  the  other  hand,  do  not  doubt  the 
fact  of  synapsis,  and  take  issue  only  with  the  parasynaptic  mode 
of  conjugation.  Concerning  the  latter  Haecker's  latest  expres- 
sion of  opinion  is  as  follows: 

Vielmehr  hat  sich  in  mir  ....  die  Ueberzeugung  befestigt, 
dass  der  Eindruck  einer  Parallelkonjugation  im  wesentlichen  durch  die 
teilweise  Koinzidenz  zweier  voneinander  unabhdngiger  Erscheinungen 

THE  JOURNAL  OP   EXPERIMENTAL  ZO6LOGT,  VOL.  13,   NO.  3 


394  EDMUND  B.  WILSON 

hervorgerufen  kann,  namlich  erstens  eines  mehr  zufalligen  oder,  besser 
gesagt,  selbsverstandlichen  teilweisen  Parallelismus  der  Fdden,  wie  er 
durch  die  in  der  Synapsisphase  bestehende  polar e  Anordnung^  derKern- 
substanzen  bedingt  wird,  und  zweitens  einer  verfruhten,  bei  den  ein- 
zelnen  Objekten  und  Individuen  je  nach  dem  physiologischen  und  Kon- 
servierungszustand  bald  friiher,  bald  spater,  bald  regelmassiger  auf tre- 
tenden  primaren  Langsspaltung  ('10,  p.  185). 

Without  citing  other  zoological  critics  at  this  point,  attention 
may  be  called  to  the  increasing  tendency  now  apparent  among 
botanical  cytologists  to  reject,  or  at  least  to  restrict,  the  theory 
of  parasynapsis  held  by  Strasburger,  Allen,  Berghs,  Gregoire  and 
a  large  number  of  other  botanical  "  zygotenists,'  in  favor  of  a  telo- 
synaptic  conception  like  that  of  Farmer  and  Moore('05),  itself 
essentially  like  that  many  years  earlier  maintained  by  Haeckerand 
Riickert  among  zoologists.  Among  these  may  be  mentioned 
Mottier  ('07,  '08),  Gates  ('08,  '11),  Davis  ('09,  '11)  and  Digby 
('10).  These  observers  and  others,  though  differing  more  or 
less  as  to  the  details,  are  in 'agreement  on  the  essential  point 
that  in  some  species  at  least  the  synaptic  connection  of  the 
chromosomes  is  end  to  end,  not  side  by  side;  and  that  a  longitu- 
dinal duality  of  the  spireme-threads  at  the  synaptic  period 
(synizesis,  or  earlier)  is  either  absent,  or  if  present  is  due  either 
to  an  accidental  parallelism  or  to  a  longitudinal  splitting  compara- 
ble to  that  seen  in  the  diploid  prophases.  These  observers  are 
in  substantial  agreement  that  the  chromosomes  (if  persistent 
entities)  are  originally  arranged  in  linear  series,  and  united  end 
to  end,  in  a  spireme-thread  which  ultimately  breaks  apart  into 
bivalent  segments,  each  consisting  of  two  chromosomes  in  para- 
synaptic  union.  The  sidewise  pairing,  which  undoubtedly  occurs 
in  some  plants,  is  believed  by  Farmer  and  Moore,  Mottier,  and 
others  to  result  from  a  secondary  looping  of  these  segments,  which 
takes  place  long  after  the  synizesis  stage.  Gates,  however, 
expressly  adopts  the  view  that  synapsis  may  take  place  by  either 
method  in  different  species,  possibly  even  in  the  same  species. 

I  must  admit  that  my  own  faith  in  parasynapsis  (such  as  it 
was)  and  even  in  synapsis  itself,  was  materially  shaken  by  some 
of  the  criticisms  and  observations  that  have  just  been  indicated, 
and  that  I  took  up  the  study  of  the  question  in  a  distinctly  scepti- 


STUDIES  ON  CHROMOSOMES  395 

cal  spirit.  It  was  only  after  prolonged  and  repeated  study  of  the 
same  objects,  in  part  of  the  same  preparations,  as  those  of  the 
Schreiners  and  of  Janssens,  that  this  scepticism  gave  way  to  the 
belief  that  the  conclusions  of  these  observers  (not  to  mention 
others)  are  probably  well  founded.  I  will  not  at  this  time  pub- 
lish new  figures  or  photographs  of  these  forms  (of  which  I  have  a 
large  number,  particularly  of  Batracoseps)  but  will  here  confine 
myself  to  a  brief  statement  of  the  main  reasons  why  I  do  not  find 
it  possible  to  accept  the  adverse  criticisms  that  have  been  indi- 
cated. 

There  are  two  points  that  demand  especial  emphasis.  One  is 
the  complete  demonstration  of  the  seriation  of  the  stages  that  is 
afforded  in  the  testis  of  Batracoseps.  The  regular  and  panoramic 
progression  of  stages  from  the  spermatogonial  end  of  the  testis 
to  that  of  the  spermatids  renders  error  on  this  point  out  of  the 
question;  and  in  particular,  there  is  no  possibility  of  confusing 
the  post-synaptic  with  the  pre-synaptic  stages,  or  the  synaptic 
nuclei  with  those  of  the  early  prophases  (pro-strepsinema)  of  the 
spermatocyte-divisions.  The  second  point  of  importance  is  the 
essential  accuracy  of  the  figures  of  the  Schreiners  and  of  Janssens — 
indeed,  my  only  criticism  of  those  of  Janssens  might  be  that  the 
relations  are  often  shown  even  more  clearly  in  the  preparations 
than  in  his  figures,  perhaps  because  the  latter  were  in  some  cases 
made  from  material  not  quite  as  perfectly  fixed  as  the  best  that  has 
come  under  my  observation.  In  the  case  of  Tomopteris  I  have 
been  able  in  a  considerable  number  of  cases  to  compare  the  figures 
of  the  Schreiners  with  the  identical  nuclei  from  which  they  were 
drawn.  Here  and  there,  perhaps,  certain  details  might  be  some- 
what differently  represented  by  different  observers;  but  a  study 
of  very  numerous  cells  at  every  stage  of  the  spermatogenesis  has 
thoroughly  convinced  me  that  as  a  whole  the  figures  of  these 
authors  present  a  faithful  picture  of  what  any  observer  may  see 
in  the  preparations.  The  only  question  that  can  be  raised  seems 
to  me  therefore  to  be  a  matter  of  interpretation. 

I  think  that  any  observer,  whatever  be  his  individual  preposses- 
sion, who  will  take  the  trouble  to  study  these  preparations  thor- 
oughly, will  find  himself  compelled  to  admit  the  following  facts: 


396  EDMUND  B.  WILSON 

1.  That  the  'amphitene'  stage  (to  employ  Janssens's  appro- 
priate term  for  the  synaptic  nuclei)  is  preceded  by  one  in  which 
the  nucleus  is  traversed  by  fine,  undivided,  leptotene  threads  the 
free  ends  of  which  are  from  an  early  period  polarized  towards  the 
pole  of  the  nucleus  near  which  lie  the  centrioles. 

2.  That  from  this  pole,  during  the  amphitene  stage,  thick  and 
often  plainly  double  threads  are  formed  progressively  towards  the 
opposite  pole  near  which  (in  Batracoseps)  lies  the  chromoplast. 

3.  That  pari  passu  with  the  growth  of  the  thick  threads  the 
thin  threads  disappear  until  all  have  vanished. 

The  conclusion  is  irresistible,  and  will  hardly  be  disputed,  that 
the  thick  (pachytene)  threads  grow  at  the  expense  of  material 
supplied  by  the  thin  ones  (leptotene). 

It  is  further  indisputable  that  in  many  cases  the  thick  (and 
often  double)  threads  terminate  anti-polewards  in  two  undivided 
diverging  thin  threads  like  the  branches  of  a  F,  which  often  sepa- 
rate at  a  wide  angle  and  may  be  traced  for  a  long  distance,  some- 
times to  opposite  sides  of  the  nucleus,  as  continuous  threads. 
This  fact  may  be  seen  in  both  Tomopteris  and  Batracoseps  with  a 
clearness  that  admits  of  no  doubt.  These  F-figures  are  so  numer- 
ous, so  clear,  and  in  their  more  striking  forms  so  different  from 
anything  seen  at  other  stages  as  to  constitute  a  highly  character- 
istic feature  of  the  nuclei  at  this  particular  stage. 

Janssens,  the  Schreiners,  Gr4goire  and  others  have  with  good 
reason  insisted  on  the  fact,  seen  with  especial  clearness  in  Batra- 
coseps, that  not  more  than  two  thin  threads  are  thus  seen  diverging 
from  the  anti-poleward  ends  of  the  thick  threads.  Pick  ('07)  after 
examination  of  the  Schreiners'  preparations  of  Tomopteris,  stated 
that  he  could  sometimes  observe  more  than  two  such  diverging 
threads.  Even  Janssens  in  his  earlier  work  (with  Dumez)  on 
Plethodon,  believed  that  he  had  seen  a  similar  appearance  "Un 
chromosome  naissant  est  parfois  en  relation  avec  plusieurs  fila- 
ments, de  tel  maniere  qu-il  devient  tres  difficile  a  1'observateur 
de  faire  un  choix"  ('03,  p.  423) ;  but  in  his  later  work  on  Batra- 
coseps he  insists  that  such  is  not  the  case. 

I  have  studied  this  point  with  the  greatest  care  of  which  I  am 
capable  in  both  Tomopteris  and  Batracoseps.  In  the  former 


STUDIES  ON  CHROMOSOMES  397 

case  one  may  indeed  often  be  in  doubt,  particularly  in  the  earlier 
stages,  though  many  perfectly  clear  F-figures  are  evident.  Such 
doubtful  cases  may  however  very  well  be  due  either  to  a  confu- 
sion produced  by  threads  of  linin,  to  defects  of  fixation,  or  to 
coagulation-products  of  the  nuclear  enchylema.  Batracoseps 
seems  to  me,  however,  decidedly  more  favorable  for  study  of  this 
point  than  Tomopteris,  partly  because  of  the  much  greater  size 
of  the  nuclei,  partly  because  of  the  greater  brilliancy  of  the  pic- 
tures in  detail,  especially  evident  in  material  fixed  with  Carney's 
fluid.  At  its  best  this  method,  in  my  experience,  is  much  supe- 
rior to  Flemming's  or  Hermann's  fluids  for  study  of  this  point. 
Prolonged  search  among  the  huge  amphitene-nuclei  of  Batra- 
coseps has  failed  to  show  even  a  single  clear  case  in  which  more 
than  two  leptotene-threads  can  be  traced  into  connection  with  a 
single  pachytene.  When  the  latter  terminate  anti-polewards  in 
more  than  one  leptotene-thread  two  are  always  seen,  very  often 
diverging  like  the  branches  of  a  F;  and  these  bifid  figures  appear 
with  the  utmost  clearness  in  every  view — sidewise,  endwise  and 
obliquely.  It  is  of  course  true  that  such  bifid  figures  are  often 
not  in  evidence.  Not  infrequently  pachytene-threads  seem  to 
end  abruptly  without  connection  with  the  leptotene;  sometimes 
they  seem  to  end  in  single  leptotene-threads.  But  in  the  nature 
of  the  case  the  true  relation  of  the  latter  to  the  former  must  often 
fail  to  appear  in  the  sections.  This  may  result  from  many  causes 
— accidents  of  sectioning,  entanglement  of  the  threads,  unfavor- 
able position,  and  the  like — and  it  is  very  probable  that  in  the 
coagulation-process  of  fixation  the  delicate  thin  threads  may  often 
break  away  from  their  normal  connections.  When  allowance  is 
made  for  these  sources  of  error  it  is  in  fact  surprising  that  so  many 
demonstrative  F-figures  are  seen;  and  it  is  a  significant  fact  that 
these  figures,  though  often  bent  or  distorted,  always  show  the 
same  orientation  in  the  nucleus  with  respect  to  the  centrosome 
pole. 

That  the  F-figures  represent  the  normal  and  typical  relation  of 
the  pachytene-threads  to  the  leptotene  seems  to  me  indisputable ; 
and  I  consider  it  utterly  impossible  to  interpret  these  figures  as 
an  expression  of  a  progressive  longitudinal  splitting  of  previously 


398  EDMUND  B.  WILSON 

undivided  threads.  The  F-figures  are  not  opening  apart,  they 
are  closing  up,  as  is  placed  beyond  doubt  by  the  magnificent 
demonstration  of  the  seriation  given  in  the  testis  of  Batracoseps. 
F-figures  with  a  short  stem  and  long  arms  precede,  they  do  not 
follow,  those  with  long  stem  and  short  arms.  What  is  taking 
place  is  evidently  a  coming  together  of  the  thin  threads  side  by 
side  in  pairs  to  form  the  thick  ones — a  process  exactly,  opposite 
to  longitudinal  division.  I  do  not  hesitate,  therefore,  to  confirm 
positively  the  description  of  the  facts  given  by  Janssens  and  the 
Schreiners. 

I  desire  to  emphasize  the  striking  contrast  that  exists  between 
the  amphitene-nuclei  and  the  spermatogonial  or  other  diploid 
prophase-nuclei.  It  seems  to  me  that  Meves  goes  much  too  far 
when  he  directly  compares  the  process  of  'parallel  conjugation' 
to  the  early  fission  of  spireme-threads  in  the  diploid  nuclei  as 
described  by  Flemming  and  his  successors;  for  one  is  led  from  this 
to  suppose  that  figures  may  be  seen  in  the  two  cases  that  are 
essentially  similar.  But  no  one  can  study  the  early  spermatogonial 
prophases  in  Tomopteris  or  Batracoseps  without  being  struck 
by  the  very  great  contrast  which  they  present  to  the  amphitene- 
nuclei.14  Never  in  the  former  case,  as  far  as  I  have  been  able  to 
find,  are  the  two  halves  of  the  double  spireme-threads  seen  diverg- 
ing like  the  branches  of  a  F;  nor  have  I  been  able  to  discover  such 
pictures  in  the  early  prophases  of  other  diploid  nuclei,  such  as 
the  epithelial  and  connective  tissue  cells  of  larval  salamanders. 
Even  though  such  pictures  could  be  found,  the  amphitene  nuclei 
undeniably  offer  peculiarities  that  differentiate  them  in  the  most 

14  In  considering  this  question  it  is  necessary  to  point  out  that  the  single  figure 
of  the  amphitene  stage  that  Meves  offers  in  favor  of  his  interpretation  ('07,  text- 
figure,  p.  460)  conveys  no  real  idea  of  the  characteristic  relation  of  the  leptotene- 
threads  to  the  pachytene.  Many  pictures  similar  to  this  are  seen  in  my  own  sec- 
tions of  Batracoseps  and  Plethodon,  especially  after  fixation  by  Flemming's  fluid 
or  Hermann's,  often  also  in  inferior  preparations  from  Carney's  fluid;  but  as  a 
rule  it  is  only  in  the  best  Carnoy  preparations  that  the  exact  relations  can  be 
clearly  and  generally  seen.  It  is  evident  that  the  least  defect  due  to  fixation  or 
to  the  shrinkage  of  embedding  process  tends  to  obscure  the  leptotene-threads  and 
cause  them  to  assume  a  more  netlike  appearance. 


STUDIES  ON  CHROMOSOMES  399 

conspicuous  way  from  the  earlier  generations  of  cells  in  the  testis ; 
and  these  are  not  to  be  ignored  in  the  study  of  this  problem. 

Another  very  striking  fact  in  the  case  of  Batracoseps  is  that  the 
two  branches  of  the  Y  often  give  exactly  the  appearance  of  twist- 
ing together  to  form  the  stem — a  condition  very  clearly  shown  in 
many  of  Janssens's  figures,  though  I  do  not  find  it  mentioned  in 
the  text.  The  pictures  seen  in  Tomopteris  also  sometimes  sug- 
gest a  similar  condition,  though  less  clearly;  but  in  neither  case  am 
I  entirely  sure  of  the  case,  since  the  torsion  often  can  not  be  seen. 
A  twisting  together  of  the  longitudinal  halves  of  the  diplotene- 
threads  at  a  later  stage  ('strepsinema')  is  of  course  a  very  familiar 
fact;  but  I  can  find  only  a  few  indications  here  and  there  in  the 
literature  of  such  a  twisting  at  the  synaptic  stage.  Two  very 
definite  accounts  of  such  a  process  have  recently  been  given  by 
Agar  ('11)  in  the  case  of  Lepidosiren,  and  by  Bolles  Lee  ('11)  in 
Helix.  The  latter  author  believes  the  double  spiral  to  persist 
as  such  in  the  succeeding  pachytene  stage.  "Jamais,  a  aucune 
moment,  meme  dans  les  enroulements  les  plus  etroits  du  bouquet 
tasse",  on  ne  voit  rien  qui  puisse  faire  conclure  a  une  fusion  des 
'deux  e" laments"  (p.  70).  It  is  quite  possible  that  a  close  torsion 
of  the  threads  may  explain  the  fact  that  it  is  in  many  cases  diffi- 
cult or  impossible  to  distinguish  a  longitudinal  duality  for  a  cer- 
tain time  after  the  synaptic  process  is  completed. 

Concerning  synapsis  in  the  Orthoptera  I  can  only  speak  with 
considerable  reserve.  Most  observers  of  this  group  have  con- 
cluded that  the  longitudinal  duality  of  the  diplotene-threads  is 
due  to  a  process  of  longitudinal  splitting  (McClung  and  all  of 
his  pupils,  Sinety,  Montgomery,  Davis,  Buchner,  Jordan,  Granata, 
Brunelli)  and  only  a  few  have  attributed  it  to  parasynapsis  (Otte, 
Ge"rard,  Morse) .  The  few  observations  I  have  been  able  to  make 
on  McClung's  preparations  of  Achurum,  Phrynotettix  and  Mer- 
miria  nevertheless  lead  me  to  the  impression  that  a  side  by  side 
union  of  leptotene-threads  takes  place  here  also.  The  case  is 
however  much  less  clear  than  in  the  other  forms  since  the  polari- 
zation is  less  marked,  and  the  amphitene  stage,  though  clearly 
apparent  in  some  cases,  is  correspondingly  less  conspicuous. 


400  EDMUND  B.  WILSON 

Nevertheless,  in  these  nuclei  also  the  leptotene-threads  may  often 
be  seen  to  lie  parallel  and  in  pairs  on  one  side  of  the  nucleus,  while 
on  the  opposite  side  they  are  quite  irregular.15  .Here  too  may  be 
seen  T-shaped  figures,  in  some  cases  almost  exactly  like  those  of 
Batracoseps,  except  that  the  stem  is  more  clearly  double  and  shows 
no  indication  of  torsion.  It  may  again  be  pointed  out  that  Sut- 
ton's  observations  on  Brachystola  are  entirely  consistent  with  a 
parasynaptic  mode  of  conjugation.  I  think  therefore  that  the 
case  for  telosynapsis  in  these  animals  is  not  yet  established,  and 
that  in  spite  of  the  careful  work  of  Davis,  Brunelli  and  others, 
the  question  must  still  be  considered  open. 

As  to  the  Hemiptera,  sufficient  emphasis  has  already  been  laid 
upon  the  practical  difficulties  which  they  present.  In  Euschistus 
however,  as  described  in  Montgomery's  recent  valuable  paper 
('11)  the  difficulties  are  less  baffling  than  in  many  other  forms; 
and  he  has  had  the  advantage  of  working  with  a  species  in  which 
the  total  number  of  threads  can  be  determined  in  at  least  some  of 
the  nuclei  at  every  stage.  In  this  work  the  author,  reversing  his 
earlier  conclusions  concerning  these  insects,  definitely  accepts  the 
theory  of  parasynapsis.  As  I  have  pointed  out,  the  prophase- 
figures  in  Oncopeltus  and  Protenor  are  certainly  not  out  of  har- 
mony with  this  conclusion.  I  therefore  accept  the  probability 
of  a  side  by  side  union  in  these  animals,  though  I  think  the  possi- 
bility of  an  end  to  end  conjugation  is  not  yet  excluded. 

But  if,  now,  the  fact  of  a  side  by  side  union  of  parallel  lepto- 
tene-threads be  granted,  we  have  still  not  arrived  at  a  demon- 
stration of  parasynapsis;  for  there  are  some  very  important 
possibilities  yet  to  be  reckoned  with.  Before  such  a  demon- 
stration can  be  admitted,  we  must  first  make  sure  of  the  number 
of  separate  pre-synaptic  chromosomes  (cf .  Fick,  Meves)  and  sec- 
ondly must  exclude  the  possibility,  which  has  been  suggested  by 
several  writers,  that  the  parallel  union  is  no  more  than  a  reunion 
of  sister-threads  that  have  been  derived  by  an  earlier  longitudinal 
fission  of  a  single  thread  (or  chromosome)  and  have  subsequently 

15  This  was  also  noted  by  Davis  but  attributed  by  him  to  "  an  accidental  arrange- 
ment, which  is  more  common  near  the  pole  since  in  this  region  the  threads  are 
crowded  more  closely  together"  ('08,  p.  127). 


STUDIES  ON  CHROMOSOMES  401 

undergone  wide  separation.  Such,  for  instance,  is  the  view  of 
Brunelli  ('11)  in  an  interesting  recent  work  on  Tryxalis;  and  a 
similar  view  is  suggested  on  the  botanical  side  by  .the  recent  work 
of  Digby  ('10)  on  Galtonia,  and  of  Fraser  and  Snell  ('11)  on  Vicia. 
All  these  observers  believe  the  longitudinal  duality  of  the  spireme 
threads  at  the  synaptic  period  to  be  quite  comparable  to  that  of 
the  diploid  prophases,  and  to  be  traceable  to  a  longitudinal  split 
that  is  already  present  in  the  preceding  telophase-chromosomes 
(cf.  also  the  work  of  Dehorne  and  of  Schneider,  already  cited),  and 
these  writers  emphasize  the  fact  that  the  products  of  this  fission 
do  in  fact  separate  more  or  less  widely  as  the  nuclei  enter  the  'rest- 
ing' period.  As  to  the  subsequent  changes  Brunelli  concludes, 
"  Successivamente,  le  due  meta  longitudinals  degli  individut 
cromosomici  si  parallelizzerebbero :  donde  gli  aspetti  intermedi 
che  sono  stati  descritti  come  scissione  di  un  filo  unico,  o  come 
1'acollimento  di  due  fili  cromatici  avendi  il  valore  di  due  cromo- 
somi  (ipotesi  della  zigotenia)"  ('11,  p.  9).  It  is  evident  from 
this  how  essential  it  is  to  determine  the  number  of  pre-synaptic 
threads;  for  if  they  have  such  an  origin  as  has  just  been  indicated, 
their  number  should  be  tetraploid  (double  the  diploid),  whereas 
if  they  represent  whole  chromosomes  the  number  should  be 
diploid. 

In  the  insects  that  I  have  studied  the  pre-synaptic  stages  are  of 
especial  interest  as  affording  almost  a  demonstration  that  the  pre- 
synaptic  number  of  threads  is  the  diploid  number.  I  attach 
great  weight  to  the  history  of  the  sex-chromosomes  in  these  stages ; 
for,  owing  to  the  fortunate  circumstance  that  they  are  individu- 
ally recognizable  at  this  time,  we  can  be  perfectly  sure  that  at 
least  one  pah1  of  chromosomes  of  the  diploid  groups  is  here  repre- 
sented by  two  separate  chromosomes  that  afterwards  undergo 
synapsis.  When  we  consider  that  these  chromosomes  are  hardly 
distinguishable  from  the  other  chromatic  masses  of  Stage  b  until 
after  considerable  extraction,  that  the  latter  are  of  the  same  or 
nearly  the  same  number  as  that  of  the  spermatogonial  autosomes, 
and  that  they  give  rise  to  the  separate  leptotene-threads  that 
enter  synapsis,  we  must  admit  that  strong  ground  is  given  for 
the  conclusion  that  the  latter  are  individually  representatives  of 


402  EDMUND  B.  WILSON 

the  spermatogonial  autosomes.  In  some  at  least  of  the  objects 
I  have  examined  these  threads  are  single,  not  double;  and  I  can 
find  no  evidence  that  they  consist  or  have  consisted  of  two  inter- 
lacing spirals  or  closely  associated  halves.  In  this  respect  they 
seem  to  be  quite  like  the  spirals  that  uncoil  from  massive  bodies 
in  the  spermatogonial  prophases  in  Phrynotettix.  In  this  case 
I  can  speak  with  complete  assurance;  for  the  evidence  afforded  by 
McClung's  brilliant  preparations  of  this  form  is  absolutely  demon- 
strative that  the  spirals  are  single,  and  that  the  longitudinal  dual- 
ity is  produced  by  a  subsequent  longitudinal  split  of  the  spiral 
thread  (which  is  essentially  in  agreement  with  Janssens's  earlier 
conclusions  in  the  case  of  Triton). 

For  the  foregoing  reasons  I  accept  the  probability  that  the 
parallel  union  of  leptotene-threads  does  not  form  part  of  a  pecu- 
liarly modified  process  of  longitudinal  division,  but  should  be 
regarded  as  a  true  conjugation  or  parasynapsis  of  entire  chromo- 
somes. Apart  from  the  convincing  evidence  afforded  by  the 
sex-chromosomes,  my  observations  are  essentially  in  agreement 
with  those  of  the  Schreiners  in  regard  to  the  origin  of  the  lepto- 
tene-threads. These  observers  describe  the  latter  in  Tomop- 
teris  as  arising  from  much  thicker,  loose,  polarized  loops  of  the 
diploid  number  (18)  which  transform  themselves  into  convoluted 
threads  in  a  manner  somewhat  similar  to  that  seen  in  the  insects : 
"Nicht  selten  haben  wir  Bilder  gesehen,  die  uns  den  Eindruck 
gegeben  haben,  dass  das  Chromatin  der  lockeren  Schlingen  sich 
zuerst  zu  einem  unregelmassig  aufgebauten,  stark  gewundenen 
und  gefalteten  Bande  sammelt,  aus  dem  wieder  die  deutlich 
begrenzten  diinnen  Faden  hervorgehen"  ('06,  p.  18).  Later, 
"Die  Chromatinfadchen,  die  auf  Stadien  wie  Fig.  18  und  20 a 
hervortreten  sind,  vovon  uns  fortgesetzte  Untersuchungen  immer 
fester  tinberzeugt  haben,  in  den  breiten  aufgelockerten  Chro- 
matinbander  der  vorgehenden  Stadien  spiral  aufgerpllt  oder  zusam- 
mengefaltet  sind"  ('08,  p.  10,  italics  mine).  My  own  study  of 
the  Tomopteris  slides  gives  me  the  same  impression;  and  I  think 
it  probable  that  the  phenomenon  here  seen  is  of  the  same  nature 
as  that  which  so  clearly  appears  in  the  insects,  though  the  thick 
'  Chromatinbander'  are  here  much  less  sharply  defined.  Jans- 


STUDIES  ON  CHROMOSOMES  403 

sens's  somewhat  similar  account  of  the  pre-synaptic  stages  in  Tri- 
ton have  already  been  mentioned  (p.  368).  In  Batracoseps,  on 
the  other  hand,  there  is  as  yet  nothing  to  show  that  the  lepto-' 
tene-threads  arise  directly  from  the  irregular  and  variable  chro- 
matin-masses  that  are  seen  in  the  earlier  stages  ('protobroch' 
and  'deutobroch'  nuclei). 

Opinion  still  differs  so  widely  in  respect  to  the  pre-synaptic 
conditions  in  plants  that  its  discussion  can  hardly  be  under- 
taken here.  Overton  ('05,  '09)  and  those  who  have  adopted  the 
'prochromosome-theory'  find  the  leptotene  stage  preceded  by 
one  in  which  massive  '  prochromosomes'  are  present,  of  the  dip- 
loid  number,  and  already  showing  an  association  in  pairs  which  is 
a  forerunner  of  actual  synapsis;  but  other  observers  have  found 
no  support  for  this  view  in  the  objects  they  have  examined  (cf. 
Mottier,  '07,  '09).  It  seems  possible  that  different  species  may 
differ  in  this  respect,  as  is  certainly  the  case  in  animals. 

It  is  impossible  to  leave  this  discussion  without  mention  of  two 
additional  series  of  facts  which  lend  strong  indirect  support  to 
the  theory  of  synapsis.  One  is  the  remarkable  discovery  that  in 
the  diploid  groups  the  chromosomes  are  often  found  to  corre- 
spond two  by  two  in  respect  to  size,  as  was  first  pointed  out  by 
Montgomery  ('01)  and  Sutton.  ('02) ;  and  that  in  some  cases  the 
chromosomes  are  actually  arranged  in  pairs  according  to  their 
size.  The  latter  fact  was  also  first  described,  I  believe,  by  Mont- 
gomery ('04)  in  Plethodon,  later  in  a  number  of  the  Hemiptera 
('06) ;  and  in  the  latter  work  first  appears  the  view  that  such  an 
actual  arrangement  in  pairs  is  characteristic  of  the  diploid  nuclei 
(p.  148).  A  similar  arrangement  was  later  described  by  Janssens 
and  Willems  ('08)  in  Alytes;  and  a  most  striking,  unmistakable 
case  of  the  same  kind  was  found  by  Stevens  ('08)  in  several  of  the 
Diptera.  On  the  botanical  side  similar  facts  have  been  de- 
scribed by  Strasburger  ('05),  Geerts  ('07),  Sykes  ('09),  Miiller 
('09),  Gates  ('09),  Stomps  ('11)  and  others.  Overton,  Rosenberg, 
Lundegard,  and  others  likewise  describe  the  "prochromosomes" 
as  arranged  in  pairs  in  the  diploid  nuclei,  as  well  as  in  the  pre- 
synaptic  stages  of  the  auxocytes.  So  many  of  these  cases  have 
been  described,  some  of  them  of  quite  demonstrative  character 


404  EDMUND  B.  WILSON 

(Diptera),  that  no  doubt  can  exist  of  the  widespread  tendency  of 
the  chromosomes  to  assume  this  arrangement  already  in  the  dip- 
'loid  nuclei.  It  appears  to  me  however  that  those  authors  who 
consider  the  paired  grouping  to  be  a  general  characteristic  of  the 
diploid  nuclei  go  much  too  far.16  Not  only  are  numerous  excep- 
tions seen  in  the  case  of  individual  chromosome-pairs,  but  in  many 
cases  no  trace  of  paired  grouping  appears.  Such  exceptions  may 
readily  be  seen  in  the  figures  of  Montgomery,  Stevens,  Morrill 
and  myself  of  the  Hemiptera,  which  are  probably  unsurpassed 
among  animals  for  the  clearness  with  which  the  size-relations 
are  shown.  In  cases  where  certain  pairs  may  be  unmistakably 
recognized  (as  in  Protenor  and  other  Coreidea)  the  two  members 
frequently  show  no  constancy  of  relative  position — compare  for 
instance  the  accurate  figures  of  the  diploid  groups  of  Protenor, 
Anasa,  Alydus  and  Nezara  in  my  third  '  Study'  ('06)  or  those  of 
Morrill  ('10)  of  Archimerus,  Chelinidea,  Anasa  and  Protenor. 
But  this  does  not  in  the  least  lessen  the  significance  of  the 
remarkable  cases  that  have  been  established.  The  tendency 
towards  such  an  association  of  the  chromosomes  in  pairs  is  un- 
doubtedly widespread;  and  the  very  fact  that  it  does  not  follow 
a  fixed  order  may  be  used  as  an  argument  in  favor  of  a  conjugation 
at  the  synaptic  period.  When  the  pairing  is  already  evident  in 
the  diploid  groups  the  way  for  synapsis  has  been  prepared  in 
advance.  This  process  must  take  place  at  some  time  subsequent 
to  the  association  of  the  germ-nuclei  in  fertilization;  and  that 
such  a  process  undoubtedly  occurs  nullifies  all  the  a  priori 
objections  that  might  be  urged  against  the  possibility  of  a  cor- 
responding process  that  is  delayed  until  the  maturation-period 
is  reached. 

11  Strasburger,  for  example,  says,  "Ich  zweifle  nicht  im  geringsten  daran,  dass 
es  sich  um  eine  allgemeine  Erscheinung  in  diploiden  Kernen  dabei  handelt,  wenn 
sie  auch  nicht  immer  auffallig  ist"  ('09,  p.  90).  Gates  is  still  more  specific.  "It 
is  evident  that  the  pairing  of  the  chromosomes  is  not  brought  about  at  synapsis  or 
at  any  other  period  of  meiosis,  but  that  the  chromosomes  are  really  paired  through- 
out the  life  cycle  of  the  sporophyte  .  .  .  Synapsis  plays  no  special  part  in  the 
pairing  .  .  .  Meiosis  and  reduction  consists  essentially  in  the  segregation  of 
the  members  of  these  pairs  that  have  been  in  association  since  soon  after  fertili- 
zation" ('11,  pp.  334,  335). 


STUDIES  ON  CHROMOSOMES  405 

Lastly  may  be  mentioned  the  interesting  facts  observed  in  the 
maturation  of  hybrids  between  parental  forms  having  different 
numbers  of  chromosomes.  Well  known  as  Rosenberg's  results 
on  Drosera  are  ('04,  '09),  the  main  facts  may  be  again  outlined, 
especially  as  exactly  analogous  results  have  recently  been  reached 
by  Geerts  ('11)  in  hybrid  Oenotheras.  On  crossing  Drosera 
longifolia  (having  forty  chromosomes)  with  D.  rotundifolia  (hav- 
ing twenty  chromosomes)  the  hybrids  have  the  intermediate 
number  of  chromosomes,  thirty  (20  +  10).  In  the  first  matura- 
tion-division appear  ten  double  and  ten  single  chromosomes,  the 
former  undergoing  a  regular  division,  and  distribution  to  the  poles, 
while  the  latter  fail  to  divide,  undergo  an  irregular  distribution, 
and  often  fail  to  enter  the  daughter-nuclei.  Rosenberg's  inter- 
pretation is  that  the  ten  rotundifolia  chromosomes  conjugate 
with  ten  of  the  longifolia  ones  to  form  the  ten  bivalent  (double) 
chromosomes,  leaving  ten  longifolia  chromosomes  as  unpaired 
univalents  which  undergo  irregular  distribution.  The  results  of 
Geerts  are  exactly  analogous.  Oenothera  gigas  (twenty-eight 
chromosomes)  crossed  with  Oe.  lata  (fourteen  chromosomes) 
gives  hybrids  with  twenty-one  chromosomes  (14  +  7).  The 
first  division  shows  seven  double  (bivalent)  and  seven  single 
(univalent)  chromosomes;  and,  as  in  Drosera,  the  bivalents  divide 
equally  and  symmetrically,  while  the  univalents  wander  irregularly 
along  the  spindle  and  often  fail  to  enter  the  daughter-nuclei. 
His  interpretation  is  the  same  as  that  of  Rosenberg.17  If  cor- 
rect, these  results,  indirect  though  they  be,  constitute  almost  an 
experimental  demonstration  of  both  synapsis  and  reduction. 

In  summing  up,  it  is  my  opinion  that  in  spite  of  all  the  apparent 
contradictions  and  conflict  of  opinion  concerning  the  modus  oper- 

17  Gates  however  ('09)  in  an  earlier  study  of  the  same  hybrid  examined  by 
Geerts,  was  led  to  quite  different  results,  concluding  that  half  the  pollen  cells 
receive  10  chromosomes  and  half  11.  I  can  not,  however,  find  evidence  in  his  paper 
to  sustain  his  conclusion  that '  'there  is  not  here  a  pairing  and  separation  of  homolo- 
gous chromosomes  of  maternal  and  paternal  origin"  (op.  cit.,  p.  195).  Gates  gives 
no  account  of  the  exact  mode  of  distribution  of  the  chromosomes  in  the  hetero- 
typic  division,  but  only  the  end  result.  This  result  would  however  follow  if  ex- 
actly such  a  pairing  and  disjunction  took  place  as  is  described  by  Geerts,  provided 
the  remaining  chromosomes  also  underwent  an  approximately  equal  distribution. 
It  remains  therefore  to  be  seen  whether  the  apparent  contradiction  of  results  is 
real. 


406  EDMUND  B.  WILSON 

andiof  synapsis,  the  cumulative  force  of  the  evidence  in  favor  of  the 
fundamental  fact  is  irresistible.  This  question  is  not  to  be  judged 
alone  by  the  study  of  any  one  of  its  single  phases.  The  whole 
extensive  series  of  facts  must  be  reckoned  with ;  and  despite  their 
variations  in  detail  the  data  are  too  consistent  in  their  fundamental 
aspects,  to  be  explained  away.  On  the  other  hand,  it  is  obvious 
that  the  problem  as  to  how  the  parental  chromatin-homologues 
become  definitely  associated  in  pairs  is  still  far  from  a  definitive 
solution.  We  should  certainly  expect  a  phenomenon  so  funda- 
mental to  follow  everywhere  the  same  type;  but  I  am  in  agree- 
ment with  the  opinion  that  has  been  expressed  by  some  other 
writers  (cf.  Gates,  '11)  that  the  particular  mode  of  union  may  be  of 
subordinate  significance.  In  accepting  the  main  conclusions  of 
the  Schreiners  and  of  Janssens  in  regard  to  Tomopteris  and  Batra- 
coseps  I  do  not  mean  to  imply  that  end  to  end  conjugation,  or 
telosynapsis,  may  not  also  take  place  in  other  forms.  I  hold  no 
brief  for  parasynapsis;  and  I  fully  recognize  the  weight  of  the  evi- 
dence in  favor  of  telosynapsis  recently  brought  forward  especially 
by  the  botanical  cytologists  that  have  been  cited.  The  studies  of 
King  ('07,  '08)  on  Bufo  should  also  be  emphasized  in  this  connec- 
tion. I  repeat  that  the  phenomena  seen  in  the  insects  by  no 
means  exclude  the  possibility  of  synapsis  of  this  type,  at  least 
in  some  forms.  Nearly  all  observers  are  agreed  that  the  side  by 
side  union  begins  at  or  near  the  free  ends  of  the  leptotene-threads 
and  advances  step  by  step  along  their  course.  We  can  here  see 
how  readily  the  one  mode  might  pass  into  the  other;  and  the  sug- 
gestion of  Gates  that  the  mode  of  synapsis  may  be  correlated 
with  the  shape  of  the  conjugating  elements  at  the  time  of  their 
union  seems  well  worthy  of  consideration. 

It  is  not  to  be  denied  that  the  acceptance  of  parasynapsis  in- 
volves some  very  puzzling  difficulties.  It  is,  for  instance,  hard  to 
comprehend  how  long  loop-shaped  chromosomes  can  become 
so  disposed  as  to  undergo  progressive  side  by  side  union  from  their 
free  ends  towards  the  central  points,  as  both  the  Schreiners  and 
Janssens  have  concluded.  Janssens  appears  to  recognize  this 
when  he  says:  " L'eloignement  des  filaments  (i.e.,  the  wide  diver- 
gence of  the  branches  of  the  F-figures)  avant  leur  soudure  est  un 


STUDIES  ON  CHROMOSOMES  407 

fait  tres  etonnant,  mais  .absolument  certain;"  but  he  very  justly 
adds, "  De  ce  que  nous  ne  pouvons  pas  expliquer  par  quel  mechan- 
isme  la  soudure  se  realise  dans  de  tels  cas,  il  ne  s'en  suit  rien 
centre  son  existence  indubitable"  ('08,  p.  167).  However  diffi- 
cult such  a  mode  of  union  may  seem  a  priori,  the  preparations  of 
the  Schreiners  actually  demonstrate  double  loops  that  are  united 
at  both  ends  while  widely  separate  along  their  middle  portions — 
shown  for  example  in  figs.  16,  17  and  18  of  their  paper  of  1908, 
which  accurately  represent  the  facts,  as  I  am  able  to  confirm  from 
examination  of  these  identical  nuclei  in  the  original  preparations. 
We  must  seek  to  discover  by  observation  how  the  conjugating 
loops  disentangle  themselves  from  the  apparent  chaos  of  the  lep- 
totene-spireme.  The  chaos  may  however  be  apparent  rather  than 
real.  The  interesting  facts  worked  out  by  Janssens  in  regard  to 
the  persistent  orientation  of  the  loops  in  the  pre-synaptic  stages 
of  Batracoseps  indicate  that  their  polarity  is  not  lost  at  any  time 
between  the  final  spermatogonial  anaphases  and  the  amphitene 
stage,  and  that  their  free  ends  always  converge  towards  the  cen- 
trosome.  It  seems  quite  possible  that  the  way  for  synapsis  may 
be  prepared  already  in  a  very  early  pre-synaptic  stage,  by  a  defi- 
nite regrouping  of  the  chromosomes  that  may  take  place  before 
the  leptotene  loops  are  formed  as  such.  It  is  evident  that  the 
central  portions  of  the  loops  are  constantly  shortening  as  the 
peripheral  portions  come  together  (possibly  as  a  result  of  the 
progressive  torsion  of  the  latter) .  It  seems  therefore  by  no  means 
a  hopeless  task  to  undertake  a  definite  solution  of  the  puzzle  by 
observation. 

2.  The  question  of  the  reduction-division 

The  history  of  the  sex-chromosomes  in  Oncopeltus  affords,  I 
believe,  complete  demonstration  of  the  occurrence  both  of  synap- 
sis and  of  a  true  reduction-division  in  the  original  sense- — i.e., 
of  the  disjunction  of  two  entire  chromosomes  that  have  previously 
conjugated  in  synapsis.  But,  although  this  creates  a  certain 
presumption  in  favor  of  the  occurrence  of  a  similar  process  in  case 
of  the  other  chromosome-pairs,  this  argument  must  not  be  pushed 
too  far— ^indeed,  there  is  reason  to  believe  that  in  case  of  the  ordi- 


408  EDMUND  B.  WILSON 

nary  chromosomes  ('autosomes')  the  process  may  be  different  in 
some  important  respects  from  that  seen  in  the  sex-chromosomes; 
and  we  must  not  lose  sight  of  the  wide  difference  of  behavior  in 
other  respects  that  differentiates  the  latter  from  the  former.  It  is 
possible  that  the  sharply  marked  process  of  conjugation  and 
disjunction  characteristic  of  the  sex-chromosomes  and  m-chro- 
mosomes  may  be  correlated  with  their  specific  functional  relations. 
The  case  for  the  autosomes  must  therefore  rest  upon  their  direct 
study,  and  a  reduction-division  can  only  be  fully  established  by 
tracing  the  bivalents  as  double  bodies  or  'gemini'  through  every 
stage  from  the  time  of  their  conjugation  to  that  of  their  disjunc- 
tion. 

Whatever  view  of  synapsis  be  adopted,  this  is  a  difficult  task. 
If  we  take  the  view  that  the  chromosomes  are  arranged  in  linear 
series  in  a  spireme-thread  which  breaks  into  bivalent  segments 
each  consisting  of  two  chromosomes  in  telosynaptic  union  there  is 
no  guarantee,  as  far  as  I  can  see,  that  the  latter  ultimately  sepa- 
rate at  the  synaptic  point.  If  we  accept  parasynaptic  conjuga- 
tion the  difficulty  is  of  a  different  kind,  namely,  the  extremely 
close  union  of  the  conjugants  side  by  side,  which  as  nearly  all 
observers  are  agreed,  follows  upon  synapsis.  The  most  that  has 
been  asserted  by  these  observers  has  been  that  evidence  of  longi- 
tudinal duality  can  always  be  seen  in  some  of  the  bivalents  at 
every  stage.  Without  reviewing  all  these  cases,  I  will  only  recall 
that  in  the  case  of  Batracoseps,  for  example,  Janssens  says, 
"  Pendant  le  long  stade  du  bouquet,  les  anses  sont  simples  et  par 
aucune  me'thode  cytologique  nous  ne  parvenons  a  y  reconnaitre 
la  moindre  trace  de  dualite"  ('05,  p.  401).  In  case  of  Tomopteris 
the  Schreiners  admit  that  at  a  period  shortly  following  synapsis 
no  longitudinal  division  can  be  seen  in  the  pachytene-threads; 
and  these  authors  are  compelled  to  fall  back  upon  indirect  evi- 
dence in  support  of  their  conclusion  that  the  duality  is  not  really 
lost.  Again,  in  Montgomery's  recent  work  on  Euschistus  ('11) 
he  expresses  the  conviction  that  there  is  no  valid  evidence  of  any 
actual  fusion  between  the  conjugants;  yet  in  point  of  fact  states 
that  after  the  completion  of  synapsis  "The  autosomal  loops 
(bivalents),  are  in  one-half  the  normal  number  and,  for  the  most 


STUDIES   ON  CHROMOSOMES  409 

part,  each  of  them  appears  solid  and  undivided."  The  most  that 
can  be  said  appears  to  be  that  "in  the  greater  number  of  cases 
there  is  to  be  seen  in  each  geminus  at  least  traces  of  a  clear  space 
which  marks  the  original  point  of  meeting  of  two  univalents" 
('11,  p.  738).  It  seems  to  me  that  this  is  hardly  a  sufficient  basis 
for  so  important  a  conclusion.  Many  similar  statements  might 
be  cited  from  other  authors.  On  the  other  hand  some  very  com- 
petent observers  not  only  find  no  evidence  of  duality  in  the  early 
post-synaptic  bivalents  but  definitely  conclude  that  the  con- 
jugants  completely  fuse  to  form  'zygosomes'  or  'mixochromosomes' 
(e.g.,  Vejdovsky,  '07,  for  the  Enchytraidae,  Bonnevie,  '08,  '11, 
for  Allium  and  other  forms,  Winiwarter  and  Saintmont,  '09,  for 
the  cat). 

In  the  case  of  Batracoseps  I  can  fully  confirm  Janssens's  state- 
ment that  no  evidence  of  longitudinal  duality  can  be  seen  in  the 
pachytene-loops  throughout  the  greater  part  of  the  growth- 
period,  even  in  the  most  perfect  preparations,  and  after  various 
modes  of  fixation,  staining  and  extraction.  It  is  only  in  the  earlier 
period  that  the  duality  appears,  and  then  often  only  here  and  there 
and  in  a  small  portion  of  the  thread.  It  is  the  same  in  Tomop- 
teris.  The  longitudinal  cleft  often  so  clearly  seen  at  the  time  of 
synapsis  seems  soon  to  disappear,  so  that  for  a  time  nothing  can 
be  seen  in  the  pachytene-threads  to  indicate  their  bivalent  nature. 
Theoretically  it  is  of  course  quite  possible  that  this  appearance  is 
deceptive,  and  that  the  two  elements  are  in  reality  always  dis- 
tinct; but  if  we  resort  to  theory  an  equally  strong  case  can  be 
made  out,  I  think,  in  favor  of  partial  or  complete  fusion.  It 
seems  at  any  rate  certain  that  in  some  of  the  most  favorable 
material  thus  far  found  among  animals,  synapsis  is  followed  by  a 
union  so  intimate  that  no  adequate  evidence  of  duality  is  after- 
wards seen  until  the  diplotene  stage  is  reached  in  the  prophases 
of  the  first  division.  It  is  very  possible  that  this  may  be  due  in 
some  cases  to  a  close  twisting  together  of  the  threads  (cf.  p.  399) 
but  it  would  hardly  be  safe  to  accept  such  an  explanation  at 
present. 

I  am  myself  inclined  to  accept  the  evidence  at  its  face  value,  and 
to  conclude  that  parasynapsis  is  followed  by  at  least  a  partial 

THE  JOURNAL  OF  EXPERIMENTAL  ZOdLOOT,  VOL.    13,   NO.   3 


410  EDMUND  B.  WILSON 

fusion  of  the  two  conjugants;and  that  the  synaptic  process  involves 
not  merely  an  association  of  the  chromosomes  to  form  'gemini' 
but  a  process  of  reconstruction  which  may  profoundly  change 
their  composition  (cf.  Boveri,  '04).  I  am,  however,  by  no  means 
in  agreement  with  those  writers  who  for  a  similar  reason  would 
reject  in  toto  the  conception  of  the  reduction-division  in  the  case 
of  these  chromosomes.  Very  important  evidence  upon  this  point 
is  afforded  by  the  contrast  in  structure  and  behavior  between  bival- 
ents  and  univalents  in  the  maturation-divisions ;  and  this  has  not  yet 
received  sufficient  attention  on  the  part  of  writers  on  this  general 
subject.  In  the  first  place,  it  is  a  rule,  without  exception  so  far  as  I 
am  aware,  that  univalent  chromosomes  divide  but  once  (of  course 
equationally)  in  the  course  of  the  two  maturation-divisions,  while 
bivalents  divide  twice.  The  additional  division  in  case  of  the 
bivalents  must,  therefore,  be  in  some  manner  a  consequence  of 
synapsis.  In  the  second  place,  the  difference  between  univalents 
and  bivalents  is  often  clearly  displayed  in  a  characteristic  differ- 
ence of  structure  in  the  prophases.  In  the  insects  that  I 
have  studied  the  former  are  always  bipartite  bodies,  the  latter 
often  quadripartite — obviously  in  preparation  for  a  single  divi- 
sion in  the  former  case,  for  two  divisions  in  the  latter.  The  best 
examples  of  these  facts  are  offered  by  the  sex-chromosomes;  but 
they  are  also  exhibited  by  the  w-chromosomes  of  Hemiptera,  and 
by  certain  anomalies  sometimes  seen  in  the  autosomes. 

Perhaps  the  most  striking  of  these  cases  is  that  of  the  X-chromo- 
some  because  of  the  different  conditions  seen  in  different  species. 
In  some  forms  this  chromosome  is  accompanied  by  a  synaptic 
mate  (the  F-chromosome)  with  which  it  unites  to  form  a  bivalent 
before  the  spermatocyte-divisions  (Coleoptera,  Diptera) ;  in  other 
species  X  and  Y  divide  as  separate  univalents  in  the  first  division 
and  afterwards  conjugate  (many  Hemiptera) ;  while  in  still  others 
Y  is  missing  and  X  is  always  univalent.  In  the  first  case  the  XY- 
bivalent  'divides'  in  both  spermatocyte-divisions — reductionally 
in  the  first,  equationally  in  the  second,  as  may  be  clearly  seen 
because  of  the  inequality  of  X  and  Y  (Stevens).  In  the  second 
case  (e.g.,  Oncopeltus  Lygaeus)  this  order  is  reversed,  the  first 
division  being  of  course  equational,  the  second  reductional.  In 


STUDIES  ON  CHROMOSOMES  411 

the  third  case  X  divides  in  but  one  spermatocyte-division  (either 
the  first  or  the  second  according  to  the  species)  and  in  the  other 
passes  undivided  to  one  pole.  It  is  here  perfectly  clear,  as  has 
been  urged  by  McClung  ('01,  '02)  and  myself  ('05  c),  that  the 
failure  to  divide  in  one  division  is  due  to  the  absence  of  a 
synaptic  mate;  and  it  is  thus  rendered  doubly  certain  that  in  case 
of  the  .XT-pair  but  one  true  division  (an  equational)  takes  place, 
the  other  'division'  (reductional)  being  merely  the  separation  of 
the  synaptic  mates.  This  is,  I  think,  a  conclusive  demonstra- 
tion in  the  case  of  these  chromosomes  of  the  reality  (1)  of  the  con- 
ception of  bivalence,  (2)  of  the  reduction-division  in  its  original 
and  unmodified  sense.  That  these  conclusions  are  not  limited 
to  the  sex-chromosomes  is  shown  by  the  ra-chromosomes  of  the 
Hemiptera,  which  have  no  relation  to  sex  as  far  as  known.  In  this 
case  conjugation  (synapsis)  is  usually  delayed  until  the  last  pos- 
sible moment  before  the  first  division.  Their  separation,  which 
immediately  ensues,  is  again  a  true  reduction-division  of  the  m- 
bivalent ;  but  what  we  here  call  a  '  division'  is  obviously  not  prop- 
erly such  but  only  the  disjunction  of  two  distinct  bodies  that 
have  but  just  come  into  momentary  contact.  In  this  case,  as 
in  that  of  the  -XT-pair,  the  term  reduction  'division'  is  a  mis- 
nomer. But  one  actual  division  takes  place,  the  equation-divi- 
sion. This  is  fully  borne  out  by  the  interesting  anomaly  that  I 
described  in  a  single  individual  of  Metapodius  in  my  sixth  'Study,' 
consisting  in  the  presence  of  three  m-chromosomes  instead  of 
two.  Here  all  three  uniformly  couple  to  form  a  triad  element  in 
the  first  division,  which  immediately  breaks  up  into  its  compo- 
nents, of  which  one  passes  to  one  pole  and  two  to  the  other.  In 
the  second  division  all  three  divide  equationally,  so  that  half 
the  spermatids  receive  but  one  w-chromosome  half  two.  This 
is,  of  course,  exactly  in  accordance  with  expectation;  and  it  is  a 
remarkable  fact  that  the  two  m-chromosomes  that  are  present  in 
half  the  secondary  spermatocytes  do  not  disjoin  but  divide  equa- 
tionally, as  they  should. 

The  case  of  the  autosomes  is  different,  owing  to  the  intimate 
union  and  possible  fusion  that  follows  synapsis;  and  it  seems  prob- 
able that  the  reduction-division  must  here  be  regarded  in  a  differ- 


412  EDMUND  B.  WILSON 

ent  light.  The  history  of  these  chromosomes  as  contrasted  with 
that  of  those  just  considered,  nevertheless  affords  some  important 
evidence  bearing  on  this  question.  As  has  been  stated,  the  pro- 
phase-bivalent  is  of  quadripartite  composition  (though  this  may 
fail  to  become  visible  until  the  later  prophases)  while  thepro- 
phase-univalent  is  bipartite.  This  has  already  been  emphasized 
in  the  case  of  Oncopeltus,  but  may  studied  to  still  greater  advan- 
tage in  Protenor,  because  of  the  greater  size  of  the  chromosomes 
and  of  their  very  marked  individual  size-differences.  In  the  male 
of  this  form,  as  heretofore  described  by  Montgomery,  Morrill 
and  myself,  the  spermatogonial  groups  contain  thirteen  chro- 
mosomes, and  the  unpaired  ^-chromosome  is  very  nearly  twice 
the  size  of  the  largest  pair  of  autosomes  (photos.  35,  36).  In  the 
female  two  such  JT-chromosomes  are  present  (photos.  37,  38). 
In  the  male  the  X-chromosomes  of  course  remains  univalent 
throughout  the  entire  maturation-process,  while  the  large  pair  of 
autosomes  produces  a  bivalent  that  is  of  nearly  the  same  size  as 
the  univalent  X.  The  latter  is  at  every  period  distinguishable. 
In  the  earlier  stages  of  the  growth-period,  when  the  autosomes  are 
in  a  diffuse  and  lightly-staining  condition,  it  remains  compact,  in 
the  form  of  a  somewhat  elongate  vermiform  body,  that  is  closely 
coiled  about  or  within  a  plasmasome  to  form  a  rounded  "chromo- 
some-nucleolus  the  true  nature  of  which  only  appears  after  con- 
siderable extraction  (cf.  Montgomery,  '01,  fig.  127).  In  smears 
(as  is  the  rule  among  the  Hemiptera)  the  plasmosome  usually 
collapses,  setting  free  the  X-chromosome,  when  its  rod-like  form 
becomes  clearly  apparent  (photo.  39).  In  later  stages  it  shortens, 
thickens  and  splits  lengthwise,  so  as  to  appear  in  the  prophases 
as  a  rather  short,  thick  rod,  very  plainly  split  (figs.  120-131,  photos. 
40  to  51).  The  large  bivalent,  on  the  other  hand,  first  becomes 
recognizable  in  the  prophases,  as  the  process  of  condensation  occurs, 
when  it  may  be  studied  to  the  best  advantage  in  smear-preparations, 
in  which  all  the  chromosomes  are  spread  out  in  one  plane. 

In  such  preparations,  of  which  I  have  a  large  number,  the  large 
bivalent  is  invariably  distinguishable  by  its  large  size — nearly 
twice  that  of  any  of  the  others;  and  we  thus  have  opportunity  to 
compare  it  accurately,  side  by  side  in  the  same  nucleus,  with  a 


STUDIES  ON  CHROMOSOMES  413 

univalent  chromosome  (X)  of  the  same  size.  It  is  most  inter- 
esting to  observe  in  Protenor  the  gradual  emergence  of  the  biva- 
lents  from  the  confused  nuclear  threadwork  of  Stage  g.  Early 
stagesof  the  process  are  seen  infigs.  115  to  117,  which  clearly  demon- 
strate (1)  that  the  threads  do  not  constitute  a  continuous  spireme, 
but  are  separate,  (2)  that  they  do  not  lie  side  by  side  in  pairs  to  form 
a  diplotene,  but  are  single  and  undivided.  Figs.  118  and  119  show 
two  nuclei,  only  a  little  later  than  the  preceding,  in  which  all  the  bi- 
valents  are  clearly  seen  and  the  large  one  is  perfectly  evident.  It 
is  of  course  only  now  and  then  that  a  nucleus  in  this  stage  can  be 
found  in  which  all  the  chromosomes  are  thus  clearly  distinguish- 
able; the  bivalents  are  still  so  extended  and  irregular  as  to  present 
a  hopelessly  confused  picture  in  sections,  and  very  frequently  also 
in  smears.  It  is  however  my  firm  belief  that  the  bivalent-figures, 
intricately  entangled  though  they  are,  are  already  quite  distinct 
at  least  as  early  as  fig.  115,  and  I  do  not  hesitate  to  accept  this 
as  probable  for  the  still  earlier  and  more  confused  nuclei  that 
precede. 

From  the  latter  part  of  Stage  h  (i.e.,  figs.  118,  119)  every  step 
may  readily  be  followed  as  the  chromosomes  continue  to  con- 
dense, contract  and  increase  in  staining  capacity.  Of  the  innu- 
merable nuclei  showing  these  stages  in  my  smear-preparations  a 
few  are  shown  in  figs.  120  to  131,  and  in  photos.  40  to  51.  As 
these  figures  show,  the  typical  number  of  separate  chromatin- 
bodies  is  eight,  which  may  however  be  reduced  to  seven  by  the 
coupling  of  the  two  smallest  (w-chromosomes,  figs.  124,  129, 
130,  photos.  45,  46,  48),  or  in  certain  rare  abnormalities  may  be 
increased  to  more  than  eight  (fig.  128,  photo.  43).  Of  these 
eight,  three  are  univalent,  namely,  the  two  smallest  (m-chromo- 
somes)  and  the  large  ^L-chromosomes,  the  latter  always  distin- 
guishable by  its  more  compact  consistency,  greater  staining  capac- 
ity, rod-like  form,  and  simple  longitudinal  split. 

The  remaining  five  are  the  bivalent  autosomes,  which  from  the 
beginning  have  the  same  forms  as  in  Oncopeltus — i.e.,  double 
crosses,  double  V's,  or  longitudinally  split  rods  which  sooner  or 
later  develop  a  transverse  suture  at  their  middle  points  and  thus 
are  plainly  seen  to  be  of  quadripartite  nature. 


414  EDMUND  B.  WILSON 

A  particular  interest  attaches  to  the  striking  and  constant  con- 
trast between  the  X-chromosome  and  the  large  bivalent;  I  do 
not  here  refer  to  their  conspicuous  difference  in  texture  and  stain- 
ing capacity  in  the  earlier  stages  but  to  a  characteristic  difference 
in  morphological  composition  that  persists  up  to  the  verymeta- 
phase  of  the  first  division.  The  JT-chromosome  is  always  bipar- 
tite— a  simple  rod,  longitudinally  split.  Sometimes  it  is  curved, 
sometimes  slightly  constricted  at  the  middle  point,  sometimes 
irregular  in  form;  but  many  of  these  variations  are  evidently 
quite  accidental.  It  never  shows  the  least  approach  to  the  double 
cross  form  nor  does  a  transverse  suture  at  the  middle  point  make 
its  appearance.  In  the  final  prophase  it  enters  the  spindle  at 
right  angles  to.  the  latter,  and  undergoes  a  longitudinal  division 
(figs.  132  to  134,  cf.  Montgomery,  '01,  '06,  Wilson,  '11  a).  The 
large  bivalent,  on  the  other  hand,  is  always,  sooner  or  later,  a 
quadripartite  body.  In  most  cases  it  forms  a  fine  double  cross, 
of  the  same  type  as  that  already  described  in  case  of  Oncopeltus, 
and  showing  the  same  variations.  All  gradations  are  found,  in 
different  nuclei,  on  the  same  slide,  between  forms  in  which  the 
arms  of  the  cross  are  equal  (figs.  120,  122, 123,  photos.  40,  41,  49) 
and  those  in  which  one  pair  (the  'lateral 'arms)  are  but  just 
perceptible  (figs.  121,  124,  125).  In  some  cases,  comparatively 
rare,  the  lateral  arms  are  quite  wanting,  and  the  bivalent  appears 
as  a  longitudinally  split  rod  (figs,  126,  129,  130)  but  in  these  forms 
a  distinct  transverse  cleft  or  suture  is  often  seen  at  the  middle 
point;  and  in  a  few  cases  the  rod  is  sharply  bent  at  this  point  to 
form  a  F-shaped  figure  (fig.  127).  Sometimes  the  transverse 
cleft  is  not  seen  in  the  earlier  stages :  but  always  in  the  later  pro- 
phases 'it  becomes  evident  (figs.  129,  130,  131,  photos.  48,  51). 
In  these  stages,  as  in  Oncopeltus,  the  lateral  arms  of  the  double 
crosses  sooner  or  later  disappear,  being  apparently  progressively 
drawn  in  to  the  axial  arms,  and  in  their  place  appears  first  a 
transverse  suture  and  later  a  constriction  across  which  the  first 
division  takes  place.  In  the  early  metaphases  the  large  bivalent 
shows  two  extreme  types,  connected  by  all  intermediate  transi- 
tions. At  one  extreme  are  ring-like  tetrads  (fig.  133)  which  are 
evidently  derived  from  crosses  by  shortening  of  the  arms  and  per- 


STUDIES  ON  CHROMOSOMES  415 

sistence  of  the  central  opening.  At  the  other  extreme  are  short 
tetrad-rods  (fig.  132)  which  clearly  show  two  division-planes  at 
right  angles  to  each  other.  These  forms  may  be  derived  either 
from  the  more  elongate  rod-like  forms  of  earlier 'stages  or  from 
crosses  by  disappearance  of  the  lateral  arms.  In  both  types  the 
quadripartite  structure  is  unmistakable.  The  large  bivalent 
ultimately  assumes  a  dumb-bell  form,  with  its  longer  axis  parallel 
to  that  of  the  spindle-axis,  and  undergoes  a  'transverse'  division; 
while,  as  already  stated,  the  X-chromosome  always  enters  the 
spindle  with  its  long  axis  transverse  to  that  of  the  spindle,  and 
undergoes  longitudinal  division  (figs.  132,  134;  see  also  Wilson, 
'11,  fig.  9,  Montgomery,  '01,  figs.  136,  .138). 

No  observer  who  studies  these  nuclei  attentively  can  fail  to  be 
struck  by  the  remarkable  difference  between  the  large  bivalent 
and  the  large  X-univalent.  Its  explanation  is  obvious ;  the  former 
is  preparing  to  divide  twice,  the  latter  once,  in  the  course  of  the 
two  maturation-divisions.  But  this  does  not  yet  touch  the  root 
of  the  matter.  We  have  still  to  ask  why  two  chromosomes  of 
equal  size  in  the  same  nucleus  differ  so  widely  in  respect  to  their 
mode  of  division.  The  reply  is  again  obvious.  It  is  because  one 
of  them  has  double  the  chromosomic  value  (or  valence)  of  the 
other,  the  bivalent  representing  two  chromosomes  of  the  original 
diploid  groups,  while  the  univalent  represents  but  one.  This 
conclusion,  which  is  hardly  more  than  a  statement  of  fact,  is 
confirmed  by  a  very  interesting  anomaly  shown  in  fig.  128,  and  in 
photo.  43.  This  nucleus  contains  nine  chromosomes,  and  the 
large  bivalent  is  absent  as  such,  while  in  its  place  appear  two  sepa- 
rate and  equal  chromosomes  of  half  its  size  (B,  B  in  the  figure), 
while  the  four  smaller  bivalents  are  plainly  recognizable  (6-6) .  The 
explanation  obviously  is  that  through  an  abnormality  of  synapsis 
the  two  members  of  this  particular  pair  have  failed  to  unite  and 
have  therefore  remained  univalent.  Both  of  these  chromosomes 
have  the  form  of  simple,  longitudinally  divided  rods,  without  trace  of  a 
quadripartite  structure,  while  the  four  smaller  bivalents  all  show  the 
cross-form  (though  the  lateral  arms  are  but  slightly  developed  in 
one  of  them) .  It  may  be  surmised,  I  think,  that  if  the  later  history 
these  separate  univalents  could  be  followed  out,  they  would  be 


416  EDMUND  B.  WILSON 

found  to  divide  but  once  (equationally)  in  the  course  of  the  two 
spermatocyte-divisions,  as  in  case  of  the  X-chromosomes,  or  the 
ra-chromosomes . 

When  these  facts  are  taken  together  the  conclusion  seems  to 
me  unavoidable  that  one  of  the  divisions  of  the  bivalent  chromo- 
some (and  hence,  one  of  the  division-planes  seen  in  the  bivalent 
prophase-figures)  is  a  consequence  of  its  bivalence — i.e.,  of  its  origin 
from  two  chromosomes  instead  of  one,  of  the  original  diploid 
groups.  An  almost  conclusive  demonstration  of  this  is  given  by 
the  fact  that  when  the  X-  and  F-chromosomes  are  united  to  form  a 
bivalent  in  the  prophases,  this  body,  like  the  others,  often  shows  a 
tetrad  structure  (as  in  Brochymena,  Wilson,  '05  b,  or  Nezara, 
Wilson,  '11  a);  and  in  the  case  of  Ascaris  felis,  recently  described 
by  Edwards  ('11)  this  bivalent  has  a  double  cross-form,  closely 
similar  to  that  of  the  other  bivalents  save  for  the  inequality  of 
two  of  the  components  (op.  cit.,  fig.  2).  I  do  not  mean  to  imply 
that  either  division-plane  of  the  tetrad  represents  the  actual  plane 
of  separation  of  the  same  two  chromosomes  that  have  united  in 
synapsis;  on  the  contrary,  I  think  it  probable,  as  already  indicated, 
that  the  original  chromosomes  may  have  undergone  reconstruc- 
tion. What  may  be  said  is  that  one  division  is  independent  of 
bivalence,  the  other  a  consequence  of  it ;  and  it  is  further  clear  that 
the  former  effects  no  reduction  of  valence,  while  the  latter  does. 
Whether  we  regard  the  autosome-bivalent  as  to  its  origin  or  its 
fate,  it  has,  irrespective  of  its  relative  size,  double  the  chromo- 
somic  value  of  a  univalent  in  the  maturation-process;  and  in 
this  respect  it  is  exactly  comparable  to  an  XF-bivalent  or  an  m- 
bivalent,  in  which  one  of  the  divisions  is  demonstrably  a  reduc- 
tion-division in  the  original  sense.  This  value,  or  'valence'  is 
reduced  to  one-half  in  one  of  the  maturation-divisions.  May  we 
not  here  find  a  definition  of  the  reduction-division  that  may  be 
accepted  even  by  those  who  deny  the  individuality  of  the  chromo- 
somes, or  who  believe  that  synapsis  is  -followed  by  actual  fusion? 
We  may  define  an  equation-division  as  one  that  effects  no  reduction 
of  valence,  a  reduction-division  as  one  that  reduces  the  valence  to 
one-half.  This  conforms  exactly  to  the  observed  facts;  and  such  a 
definition  is,  I  think,  equally  consistent  with  complete  disjunc- 


STUDIES  ON  CHROMOSOMES  417 

tion  or  with  a  process  of  reconstruction  subsequent  to  synapsis. 
I  do  not  see  how  the  analysis  can  be  carried  further  without  enter- 
ing upon  theoretical  ground.  Nevertheless,  I  do  not  hesitate  to 
accept  the  probability  that  the  reduction-division,  as  thus  defined, 
involves  a  disjunction  of  chromatin-elements  of  some  kind  that 
are  involved  in  the  production  of  the  unit-factors  of  heredity 
and  that -the  Mendelian  disjunction  may  here  find  its  explanation 
(cf.  De  Vries,  '03,  Boveri,  '04).  It  seems  to  me  that  the  conclu- 
sions indicated  by  Boveri  several  years  ago  ('04)  still  remain  the 
most  probable ;  that  is  to  say,  that  the  degree  of  union  may  vary 
in  different  cases,  involving  sometimes  no  fusion  (as  is  suggested 
by  the  history  of  the  XY-pair),  sometimes  complete  fusion,  in 
other  cases  no  more  than  a  partial  exchange  of  material.  This 
point  will  be  again  touched  upon  in  connection  with  Janssens's 
theory  of  the  'chiasmatype." 

The  point  that  I  wish  here  to  emphasize  is  the  validity  of  the 
conceptions  of  bivalence  and  the  reduction-division,  which  have 
been  more  or  less  explicitly  denied  by  a  number  of  writers.  I 
accept,  of  course,  the  conclusion  of  Haecker  ('07),  Bonnevie  ('08, 
'11),  Delia  Valle  ('07),  Popoff  ('08)  and  others,  that  neither  the 
heterotypical  form  of  division  nor  an  apparent  tetrad-structure  is 
necessarily  diagnostic  of  bivalence  or  a  reduction-division.  Tet- 
rad-like chromosomes  have  been  repeatedly  described  in  somatic 
divisions  (see  especially  Delia  Valle,  Popoff,  cited  above),  and  also 
in  the  univalent  chromosomes  of  the  second  maturation-division 
a  striking  example  of  which  is  the  '  d-chromosome'  of  Nezara, 
described  in  my  seventh  'Study.'  Bonnevie,  especially,  has 
demonstrated  in  Nereis  and  other  forms  the  very  close  similarity 
of  the  somatic  chromosomes  (in  the  cleavage  of  the  egg)  to  the 
heterotype-rings  of  the  maturation-divisions.  Her  conclusion  is: 

Ich  habe  in  meinen  Objekten  nicht  nur  fur  die  Annahme  einer  Reduk- 
tionsteilung  keinen  einzigen  Beweis  finden  konnen;  meine  Untersuchung 
hat  auch  ergeben,  dass  die  friiher  in  der  heterotypischen  Natur  der  ersten 
Reifungsteilung  gesehene  Stiitze  einer  solchen  Annahme  sammtlich 
hinfallig  sind  .  .  .  Die  beiden  Reifungsteilungen  miissen  daher, 
bis  anderes  bewiesen  worden  ist,  als  Aequationsteilungen  aufgefasst 
werden  ('08,  p.  271,  '11,  p.  241). 


418  EDMUND  B.  WILSON 

Again,  from  the  existence  of  tetrad-shaped  chromosomes  in  cer- 
tain somatic  divisions  of  Amphibia,  Delia  Valle  ('07)  concludes, 
"Tutte  le  precedente  formazioni  emolto  probabilmente,  quando 
esistono,  anche  quelle  della  profase  del  primo  fuso  di  matura- 
zione,  non  hanno  alcun  rapporto  con  la  reduzione  cromatica,  ma 
sono  indice  di  una  costituzione  patologica  dei  cromosomi"(!) 
Both  the  foregoing  passages,  I  think,  like  others  of  like  import  that 
might  be  cited,  overshoot  the  mark.  The  significance  of  ring- 
shaped  or  tetrad-like  chromosomes  must  be  judged  from  a  more 
critical  standpoint  than  this.  We  must  endeavor  to  discover  their 
real  meaning  in  each  case  by  the  study  of  their  origin,  their  behavior 
in  successive  divisions,  their  morphological  composition  (whether 
simple  or  compound  bodies),  their  relation  to  the  conditions 
seen  in  other  species,  and  any  other  facts  that  may  throw  light 
upon  them.  By  way  of  illustration  certain  specific  cases  may  be 
mentioned.  A  very  interesting  one  is  afforded  by  the  X-chromo- 
some  of  Syromastes  (Gross,  '04,  Wilson,  '09  a,  '09  b),  which  is  unac- 
companied by  a  synaptic  mate  (F-chromosome)  yet  forms  a  con- 
spicuous 'tetrad'  in  the  first  spermatocyte-di vision.  The  reason 
here  is  that  this  '  chromosome'  consists  of  two  components, 
which  appear  as  separate  chromosomes  in  the  spermatogonial 
groups  but  in  the  maturation-divisions  are  associated  to  form  a 
double  element  which  behaves  precisely  like  the  single  X-chromo- 
some  of  other  species,  and  obviously  corresponds  to  the  latter 
because  four  such  components  are  present  in  the  diploid  groups 
of  the  female  (Wilson,  '09  b).  The  '^chromosome'  of  Nezara 
is  a  different  but  not  less  interesting  case,  always  forming  a  con- 
spicuous 'tetrad'  in  the  second  spermatocyte-division,  but  appear- 
ing as  a  single  chromosome  in  the  diploid  groups.  These  two 
apparently  contradictory  cases  are  brought  under  the  same  point 
of  view — especially  when  compared  with  the  analogous  relations 
described  by  Morgan  ('09)  in  Phylloxera,  and  by  Browne  ('10) 
in  Notonecta — if  we  assume  that  in  each  case  the  chromosome  in 
question  was  originally  a  single  body  which  in  Nezara  shows  a 
slight  tendency  to  separate  into  two  components,  in  Syromastes 
has  already  done  so  (cf.  Wilson,  '11  a).  Neither  case  offers 


STUDIES  ON  CHROMOSOMES  419 

a  real  exception  to  the  rule;  for  such  'tetrads'  obviously  differ 
entirely  from  the  true  tetrads  of  the  maturation-processes,  which 
represent  synaptic  pairs  of  the  diploid  groups.  Perhaps  an  analo- 
gous case  is  that  of  Cyclops  and  Diaptomus,  where  Haecker  ('95, 
'02)  long  since  demonstrated  a  cross-bar  or  suture  in  each  com- 
ponent of  each  bivalent;  and  since  each  of  these  component,  is 
sooner  or  later  longitudinally  divided,  double  tetrads  or  'ditet- 
rads'  are  thus  produced.  The  work  of  Braun  ('09)  and  Matschek 
('09,  '10)  confirms  this  but,  like  that  of  Haecker  himself,  proves 
that  the  cross  suture  is  without  significance  for  the  divisions, 
since  both  the  latter  are  longitudinal  (cf.  also  Lerat,  '05);  while 
Krimmel  ('10)  has  shown  that  in  Diaptomus  the  transverse  con- 
striction or  suture  is  also  present  in  the  univalent  chromosomes  of 
the  diploid  groups  in  somatic  cells.  Haecker's  conclusion  that  the 
cross-suture  represents  the  point  of  end  to  end  synapsis  is  a  quite 
unproved,  and  I  think  very  improbable,  assumption.  In  view  of 
what  is  seen  in  Syromastes,  Notonecta,  Phylloxera  and  Nezara, 
it  seems  more  likely  that  each  univalent  chromosome  in  these 
forms  consists  of  two  closely  united  components,  of  which  the 
cross-suture  is  an  expression.  It  seems  quite  possible  that  the 
number  of  chromosomes  might  change  in  these  animals  by  com- 
plete separation  of  the  two  components  in  case  of  one  or  more  of 
the  chromosomes. 

Keeping  in  view  all  such  apparent  exceptions,  the  fact  remains 
that  the  heterotypic  (or  tetrad)  form  is  a  highly  characteristic 
feature  of  the  maturation  prophase-bivalents,  and  that  in  this 
respect  they  show  in  general  a  marked  contrast  to  the  univalents, 
here  and  elsewhere.  The  meaning  of  this  in  case  of  the  matura- 
tion-divisions is  unmistakable;  and  the  burden  of  proof  may  be 
left  with  those  whose  theoretical  prepossession  will  not  allow  them 
to  accept  the  natural  explanation  that  here  is  manifest. 

3.  The  chromosomes  and  heredity 

That  the  chromatin-substance  (more  specifically  that  of  the 
chromosomes)  plays  some  definite  role  in  determination  has 


420  EDMUND  B.  WILSON 

received  fresh  support  in  recent  years  from  several  sources.  Direct 
experimental  evidence  that  is  nearly  if  not  quite  demonstrative 
has  been  produced  through  the  work  of  Boveri  ('07)  on  multipolar 
mitosis,  of  Baltzer  ('09,  '10)  on  reciprocal  crosses  in  sea-urchins, 
and  of  Herbst  ('09)  and  Godlewski  ('11)  on  the  combined  effects  of 
artificial  parthenogenesis  and  fertilization  in  hybridization.  In- 
direct but  very  strong  evidence  has  been  given  by  the  demonstra- 
tion of  a  constant  relation  between  particular  chromosomes  and 
sex  and  especially  sex-limited  characters  (cf.  Wilson,  '11  a,  b, 
Morgan,  '11,  Gulick,  '11).  This  evidence  in  no  manner  precludes 
the  view  that  the  protoplasmic  substances  are  also  concerned  in 
determination — indeed  experimental  embryology  and  cytology 
have  produced  very  clear  evidence  that  such  is  the  case.  The 
study  of  the  nucleus,  and  especially  of  the  chromosomes,  offers 
however  one  of  the  most  available  paths  of  approach  to  a  study  of 
the  activity  of  the  germ-cells  in  determination,  and  for  a  detailed 
analysis  of  genetic  problems  in  their  cytological  aspects. 

It  has  been  widely  assumed  that  the  Mendelia'n  segregation 
depends  upon  a  disjunction  of  chrornatin-elements  in  the  reduction- 
division,  as  was  originally  suggested  by  Guyer,  Button,  Boveri 
and  Cannon.  It  has,  however,  becomes  obvious  from  the  experi- 
mental data  that  if  this  be  so,  these  elements  can  not  be  individual 
chromosomes  of  fixed  composition.  This  was  first  seen  to  follow 
from  the  fact,  now  apparently  well  determined  in  certain  cases, 
that  the  number  of  independent  allelomorph-pairs  may  be  greater 
than  the  number  of  chromosome-pairs.  More  recently  the  same 
result  is  demonstrated  by  the  work  of  Bateson  and  Punnett  ('11) 
on  'coupling'  and  ' repulsion'  in  certain  plants,  and  by  that  of 
Morgan  ('11  a)  on  sex-limited  characters  in  Drosophila,  which 
proves  that,  an  interchange  of  unit-factors  must  to  some  extent 
take  place  between  homologous  bearers  of  these  factors  in  the 
germ-cells.  The  results  of  Morgan  in  particular  nevertheless 
bring  strong  support  to  the  view  that  the  chromosomes  are  such 
bearers  of  unit-factors;  for  the  whole  series  of  phenomena  deter- 
mined in  Drosophila,  complicated  as  they  seem,  become  at  once 
intelligible  under  the  assumption  that  certain  factors  necessary 


STUDIES  ON  CHROMOSOMES  421 

for  the  production  of  the  sex-limited  characters  are  born  by 
the  X-chromosome;  and  without  this  assumption  they  are  wholly 
mysterious. 

Adopting  this  explanation,  the  history  of  certain  of  these  sex- 
limited  characters,  as  Morgan  points  out,  demands  the  further 
assumption  that  in  the  female  the  factors  for  these  characters 
may  to  some  extent  undergo  an  interchange  between  the  two 
sex-chromosomes  (here  two  .XT-chromosomes)  while  in  the  male 
such  interchange  does  not  take  place.  Such  a  difference  between 
the  sexes  finds  a  perfectly  simple  explanation  in  cases  where  the 
X-chromosome  of  the  male  has  no  synaptic  mate  (F-chromosome). 
When  a  F-chromosome  is  present — as  may  be  the  case  in  Droso- 
phila  according  to  the  cytological  observations  of  Stevens  ('08)— 
the  problem  becomes  more  complicated,  but  there  are  some  facts 
that  may  be  significant  in  this  direction.  It  is  well  known  that  in 
the  male  the  sex-chromosomes  commonly  retain  a  compact  and 
rounded  form  (as  'chromosome-nucleoli')  throughout  the  entire 
spermatogenesis,  and  in  some  cases  (Oncopeltus)  they  conjugate 
while  in  this  condition,  and  subsequently  disjoin  without  ever 
having  undergone  fusion  or  even  intimate  union,  such  as  is  so 
characteristic  of  the  autosomes  during  the  maturation-process. 
Unfortunately  the  oogenesis  is  as  yet  but  imperfectly  known; 
but  there  is  considerable  evidence  that  in  some  forms  the  sex- 
chromosomes  exhibit  a  quite  different  behavior  from  that  char- 
acteristic of  the  spermatogenesis.  My  own  observations  ('06) 
seemed  to  show  that  in  some  of  the  Hemiptera  the  sex-chromo- 
somes do  not  in  the  oocyte  retain  a  compact  form  at  the  period*  of 
synapsis,  or  in  the  early  growth-period,  and  the  later  observations 
of  Foot  and  Strobell  ('07)  show  that  the  same  is  true  for  later 
stages  of  the  germinal  vesicle.  The  work  of  Morrill  ('10)  further 
shows  that  in  the  female  the  sex-chromosomes  have  already  con- 
jugated before  the  first  oocyte-division.  These  fact  make  it 
probable  that  in  these  forms  the  sex-chromosomes  of  the  female 
show  the  same  behavior  in  synapsis  and  reduction  as  the  auto- 
somes,  and  enter  into  the  same  intimate  union  that  characterizes 
the  latter.  It  is  quite  possible  that  in  these  facts  we  may  find  an 


422  EDMUND  B.  WILSON 

explanation  of  the  difference  of  the  sexes  in  respect  to  the  inter- 
change of  sex-limited  factors  that  is  proved  to  take  place  by  the 
experimental  results.18 

There  is  no  a  priori  reason  why  such  a  process  of  free  interchange 
between  homologous  chromosomes  may  not  take  place  in  the 
somatic  or  diploid  nuclei.  It  is,  however,  far  simpler  to  assume 
that  it  occurs  during  or  subsequent  to  synapsis;  for  it  is  only  at 
this  period  that  the  homologous  chromosomes  are  intimately  and 
regularly  associated  (cf.  Boveri,  Strasburger,  '08,  '09).  It  is 
these  facts,  taken  together  with  the  cytological  evidence,  that 
lead  me  to  the  conclusion  already  indicated  that  the  synaptic 
union  results  in  a  reorganization  of  the  chromosomes,  and  that  the 
two  moieties  of  the  final  prophase-chromosomes  are  probably  not 
identical  with  the  original  conjugants.  Janssens's  theory  of  the 
chiasmatype  ('09)  gives  the  only  simple  mechanical  explanation 
thus  far  offered  as  to  how  such  an  orderly  exchange  of  materials 
may  be  effected;  and  his  conception  has  recently  been  applied  in 
an  ingenious  manner  by  Morgan  ('11  b)  to  an  explanation  of 
both  'repulsion'  and  'coupling'  in  heredity.  The  chiasma type- 
theory  has  been  criticized  by  Gre*goire  and  by  Bonnevie  on  the 
ground  that  a  strepsinema  stage  occurs  in  the  division  of  somatic 
nuclei  as  well  as  in  the  maturation-prophases,  and  that  the  orig- 
inal twisting  together  of  the  threads  is  in  some  cases  followed  by 
untwisting,  without  such  aprocessof  partial  fusion  and  subsequent 
secondary  splitting  as  is  postulated  by  Janssens.  There  are  two 
replies  to  this.  One  is  that  Janssens's  theory  is  not  an  a  priori 
construction  but  a  conclusion  based  on  a  most  accurate  and  de- 
tailed study  of  the  actual  conditions  seen  in  the  prophases  of 
Amphibia  which  prove  that  such  a  process  as  he  postulates  must 
here  take  place.  The  other  is  that  there  is  considerable  evidence 
that  a  twisting  together  of  the  conjugating  threads  takes  place 
in  the  process  of  synapsis,  leading  to  a  most  intimate  union  of  the 

18  It  would,  however,  be  rash  to  generalize  this  statement  at  present,  for  the 
observations  of  Buchner  on  the  female  Gryllus  ('09)  and  those  of  Winiwarter  and 
Saintmont  ('09)  on  the  cat,  have  demonstrated  a  nucleolus-like  body  in  the  synap- 
tic and  later  stages  of  the  oocytes  which  may  be  the  .X"X"-bivalent,  though  this  is 
unproved,  and  doubt  is  thrown  upon  the  second  of  these  cases  by  the  recent  work 
of  Gutherz  ('12). 


STUDIES  ON  CHROMOSOMES  423 

two  members  of  each  pair,  and  followed  by  a  longitudinal  split. 
It  would,  no  doubt,  be  premature  definitely  to  accept  this  conclu- 
sion at  present;  but  it  seems  worthy  of  more  attentive  considera- 
tion than  it  has  yet  received. 

Turning  to  the  more  general  aspects  of  these  problems,  in  what 
sense  may  we  be  justified  in  speaking  of  the  chromosomes  as 
bearers  of  the  'determiners'  or  factors  of  determination?  I  have 
recently  outlined  in  another  place  (Wilson,  '12)  my  position  in 
regard  to  this  question,  and  will  here  only  indicate  its  most  essen- 
tial features.  The  'determiners'  or  'factors'  on  which  unit-char- 
acters depend  need  not  be  regarded  as  independent,  self-propa- 
gating germs  (pangens,  biophores,  or  the  like) ;  it  is  sufficient  for 
our  purpose  to  think  of  them  in  a  more  vague  way  as  only  specific 
chemicals  entities  of  some  kind.  However  they  are  conceived  in 
this  regard,  it  would  be  a  fundamental  error  to  regard  them  as 
'bearers'  of  the  characters  that  depend  upon  their  presence  or 
absence;  for  every  character  is  produced  as  a  reaction  of  the  germ 
considered  as  a  whole  or  unit-system.  Characters  are  'borne' 
(if  the  expression  is  permissible  at  all)  by  this  system  as  a  whole; 
and  the  'unit-factors'  or  'determiners'  postulated  by  students  of 
genetics  need  be  considered  only  as  specific,  differential  factors 
of  ontogenetic  reaction  in  a  complex  organic  system.  Many 
'unit-characters'  are  known  to  depend  upon  a  number  of  such 
unit-factors,  in  some  cases  probably  upon  a  large  number;  and 
they  may  be  definitely  altered  this  way  or  that  by  varying  the 
particular  combinations  of  these  factors.  But  any  unit-factor 
produces  its  characteristic  effect  only  in  so  far  as  it  forms  a  part 
of  a  more  general  apparatus  of  ontogenetic  reaction  constituted 
directly  or  indirectly  by  the  organism  as  a  whole.  In  all  this, 
a  striking  parallel  exists  between  the  physical  basis  of  heredity  and 
the  complex  molecular  groups  of  organic  substances  such  as  the 
proteins.  The  relation  of  the  '  determiners'  to  the  qualities  of  the 
organism  considered  as  a  whole  may  be  compared  to  that  which 
exists  between  the  protein  'Bausteine'  and  the  qualities  of  the 
protein  molecular  group  as  a  whole. 

"Just  as  the  qualities  of  a  particular  protein  may  be  definitely  altered 
by  the  addition,  subtraction  or  the  substitution  one  for  another  of  parti- 


424  EDMUND  B.  WILSON 

cular  side-chains  or  molecular  'Bausteine,'  so  the  addition,  subtraction 
or  substitution  of  particular  'determiners'  or  'factors'  in  the  zygote 
calls  forth  specific  responses  that  lead  to  the  production  of  corresponding 
characters.  The  reasoning  that  applies  to  the  first  of  these  cases  seems 
equally  applicable  to  the  second.  No  one,  I  suppose,  would  hold  in  the 
first  case  that  the  particular  molecular  groups  or  '  Bausteine'  concerned 
in  the  change  are  'bearers  of  (i.e.,  are  alone  responsible  for)  the  result- 
ing new  qualities.  The  qualities  of  any  protein,  as  Kossel  has  recently 
urged,  belong  to  the  molecule  as  a  whole,  and  are  not  to  be  regarded  as 
the  sum  of  the  qualities  of  its  constituent  'Bausteine.'  Why  should  we 
regard  in  a  different  light  the  'determiners'  (chemical  substances?) 
concerned  in  the  second  case?  They  are,  clearly,  not  to  be  regarded  as 
'bearers'  or  'physical  bases'  of  the  characters  which  depend  upon  their 
presence  or  absence.  They  are,  I  repeat,  only  differential  factors  of 
ontogenetic  reactions  that  belong  to  the  germ  considered  as  a  whole  or 
unit-system"  (Wilson,  '12). 

Kossel  ('12)  makes  the  pregnant  remark  that  every  peculiarity  of 
the  species  and  every  occurrence  affecting  the  individual  may  be 
indicated  by  special  combinations  of  protein  'Bausteine.'  The 
facts  lead  us  to  seek  for  such  compounds  (substances)  in  the  chro- 
matin  or  the  chromosomes.  It  can  hardly  be  said  that  even  a 
beginning  has  been  made-in  the  chemical  investigation  of  the  dis- 
tribution of  the  chromatin-substances  within  the  nucleus.  Cyto- 
logically,  however,  a  long  series  of  the  most  significant  facts  have 
been  made  known  in  respect  to  their  groupings  and  modes  of  dis- 
tribution. Evidence  steadily  accumulates  that  these  processes 
are  perfectly  ordered;  and  the  fact  is  now  more  than  ever  evident 
that  they  run  parallel  to  the  factors  of  determination  and  heredity. 
There  has  been  a  disposition  on  the  part  of  certain  writers  of 
late  to  minimize  the  definite  order  of  the  morphological  trans- 
formations of  the  nucleus  (cf .  Fick,  '07,  Delia  Valle,  '09) ;  and 
these  authors,  among  others,  have  undoubtedly  helped  to  create 
an  impression  that  these  phenomena,  particularly  as  regards  the 
chromosomes,  are  too  vague  and  fluctuating  to  afford  trustworthy 
results  on  the  side  of  cytological  research.  I  believe  this  to  be  a 
backward  step,  though  I  am  very  ready  to  admit  the  service  to 
accuracy  of  observation  that  may  be  rendered  by  so  critical  and 
sceptical  a  spirit.  Plastic,  and  in  some  respects  variable  like 
other  biological  phenomena,  these  processes  undoubtedly  are;  but 
the  more  one  studies  them  in  detail  the  stronger  grows  the  con- 


STUDIES  ON  CHROMOSOMES  425 

viction  of  an  exact  order  that  underlies  their  superficial  fluctua- 
tions, and  one  of  which  the  main  outlines  are  steadily  becoming 
clearer. 

It  appears  to  me  that  many  of  the  recent  cytological  advances 
support  the  view  that  the  true  key  to  this  order  was  found  by 
Flemming  when  he  chose  the  word  'mitosis'  ('82),  and  by  Roux 
in  his  attempt  .to  find  the  essential  meaning  of  the  mitotic  process 
('83).  This  is  well  illustrated  by  the  pre-synaptic  phenomena 
that  have  here  been  considered,  and  by  the  increasing  body  of 
observations  that  emphasize  the  importance  of  the  mitotic  trans- 
formation of  the  chromatin-substance.  It  is  difficult  to  see  what 
meaning  such  processes  can  have  if  they  do  not  involve  a  linear 
alignment  of  different  elements  (substances)  which  are  thus 
brought  into  a  particular  disposition  for  ensuing  processes  of  divi- 
sion (Roux)  or  of  paired  association  (Strasburger).19  The  practi- 
cal utility  of  such  a  conception  for  the  analysis  of  genetic  prob- 
lems has  already  become  apparent.  It  is  still  only  an  hypothesis, 
but  one  which  we  may  hope  sooner  or  later  to  see  subjected  to 
definite  experimental  test. 

March  1,  1912. 

LITERATURE  CITED 

AGAR,  W.  E.  1911  The  spermatogenesis  of  Lepidosirenparadoxa.  Quart.  Journ. 
Mic.  Sci.,  n.  s.,  no.  225,  vol.  87,  part  I. 

ARNOLD,  G.  A.  1908  The  nucleolus  and  microchromosomes  in  the  spermatogene- 
sis of  Hydrophilus  piceus.  Arch.  f.  Zellforsch.,  Bd.  2,  no.  1. 

BALTZER,  F.     1909    Ueber  die  Entwicklung  der  Echinidenbastarde  mit  besonderer 
Berticksichtigung  der  Chromatinverhaltnisse.    Zool.  Anz.,  Bd.  5. 
1910    Ueber  die  Beziehung  zwischen  dem  Chromatin  und  der  Entwick- 
lung und  Vererbungsrichtung  bei  Echinodermenbastarden.    Arch.  f. 
Zellforsch.,  Bd.  5. 

BATESON  AND  PUNNETT  1911  On  the  inter-relation  of  genetic  factors.  Proc. 
Roy.  Soc.  Eng.,  B.,  vol.  84. 

BERGHS,  J.  B.  1904  La  formation  des  chromosomes  he'te'rotypiques.  II. 
Depuis  la  sporogonie  jusqu'au  spireme  definitif.  La  Cellule,  torn.  21, 
no.  2. 

19  This  point  has  been  forcibly  urged  by  Strasburger  (see  '08,  pp.  563-568; 
'09,  pp.  95-97). 

THE   JOURNAL  OK  EXPERIMENTAL  ZOOLOGY,  VOL.  13,   NO.  3 


426  EDMUND  B.  WILSON 

BONNEVIE,  K.      1907    'Heterotypical'  mitosis  in  Nereis  limbata.     Biol.  Bull., 
vol.  13,  no.  2. 

1908  a    Chromosomenstudien,  I.  Chromosomen  von  Ascaris,  Allium 
und  Amphiuma.     Arch.  f.  Zellforsch.,  Bd.  1. 

1908  b    Chromosomenstudien,  II.    Heterotypische  Mitosen  als  Rei- 
fungscharakter.     Ibid.,  Bd.  2. 

1911    Chromosomenstudien,   III.    Chromatinreifung  in  Allium  cepa. 

Ibid.,  Bd.  6. 
BOVERI,  TH.     1904    Ergebnisse  iiber  die  Konstitution  der  chromatischen  Sub- 

stanz  des  Zellkerns.    Jena. 

1907    Zellen-Studien,  VI.     Die  Entwicktung  dispermer  Seeigel-Eier. 

Ein   Beitrag   zur    Befruchtungslehre    und    zur    Theorie   des   Kerns. 

Jena. 
BRAUN,  H.     1909    Die  specifische  Chromosomenzahlen  der  einheimischen  Arten 

der  Gattung  Cyclops.    Arch.  f.  Zellforsch.,  Bd.  3,  no.  3. 
BROWNE,  E.  N.     1910    The  relation  between  chromosome-number  and  species  in 

Notonecta.     Biol.  Bull.,  vol.  20,  no.  1. 
BRUNELLI,  G.     1910    La  Spermatogenese  della  Tryxalis.     I.     Divisioni  sperma- 

togoniale.    Mem.  Soc.  Ital.  Sci.,  Ser.  3,  torn.  16. 

1911     La    Spermatogenese  della  Tryxalis.     II.     Divisioni  maturative. 

Reale  Ace.  d.  Lincei,  Ser.  5a,  torn.  8. 
BUCHNER,  P.     1909    Das  akzessorische  Chromosom  in  Spermatogenese  und  Ovo- 

genese  der  Orthopteren,  zugleich  ein  Beitrag  zur  Kenntnis  der  Reduk- 

tion.    Arch.  f.  Zellforsch.,  Bd.  3. 

1910    Uber  die  Beziehungen  zwischen  Centriol  und  Bukettstadium. 

Ibid.,  Bd.  5. 
CARNOY  AND  LEBRUN.     1897,  1898,  1899    La  vesicule  germinative  et  les  globules 

polaires  chez  les  batraciens.     I,  II,  III,  La  Cellule,  torn.  12,  13,  14. 
DAVIS,  B.  M.     1909    Pollen  development  of  Oenothera  grandiflora.    Ann.  Bot., 

vol.  23. 

1910  The  reduction  divisions  of  Oenothera  biennis.     Ibid.,  vol.  24. 

1911  Cytological  studies  on  Oenothera,  III.     A  comparison  of  the 
reduction  divisions  of  Oenothera  lamarckiana  and  O.  gigas.     Ibid., 
vol.  25. 

DAVIS,  H.  S.     1908    Spermatogenesis  in  Acrididae  and  Locustidae.     Bull.  Mus. 

Comp.  Zool.  Harvard,  vol.  52,  no.  2. 
DEHORNE,  A    1911    Recherches  sur  la  division  de  la  cellule.     I.  Le  duplicisme 

constant  du    chromosome    somatique  chez  Salamandre  m'aculosa  et 

chez  Allium  cepa.    Arch.  f.  Zellforsch.,  torn.  6. 
DELLA  VALLE,  P.     1907    Osservazioni  di  tetradi  in  cellule  somatiche.     Contri- 

buto  alia  conoscenza  delle  tetradi.    Atti  Ace.  Napoli,  torn.  13. 

1909  L'organizzione  della  chromatina  studiata  mediante  il  numero  dei 
cromosomi.     Arch.  Zoologico,  torn.  4. 

1911    La  continuita  della  forme  in  divisione  nucleare  ed  il  valore  mor- 
fologico  dei  cromosomi.     Ibid.,  torn.  5. 

DIGBY,  L.     1910    The  somatic,  premeiotic  and  meiotic  nuclear  divisions  of  Gal 
tonia  candicans.     Ann.  Bot.,  vol  24. 


STUDIES  ON  CHROMOSOMES  427 

EDWARDS     1910    The  idiochromosomes  in  Ascaris  megalocephala  and  Ascaris 

lumbricoides.     Arch.  f.  Zellforsch.,  Bd.  5. 

1911     The  sex-chromosomes  in  Ascaris  felis.     Ibid.,  Bd.  7. 
FARMER  AND  MOORE    1905    On  the  meiotic  phase  (reduction  divisions)  in  animals 

and  plants.     Quart.  Journ.  Mic.  Sci.,  vol.  48,  no.  4. 
FICK,   R.     1907    Vererbungsfragen,  Reduktions-  und  Chromosomenhypothesen, 

Bastard-Regeln.  Merkel  u.  Bonnet's  Ergebnisse,  Bd.  16. 

1908    Zur  Konjugation  der  Chromosomen.     Arch.  f.  Zellforsch.,  Bd.  1. 
FOOT  AND  STROBELL    1907    The  nucleoli  in  the  spermatocytes  and  germinal  vesi- 
cles of  Euschistus  variolarius.     Biol.  Bull.,  vol.  16. 
FRASER  AND  SNELL    1911    The  vegetative  divisions  in  Vicia  faba.    Ann.  Bot., 

vol.  25,  c. 
GATES,  R.  R.     1907    Pollen  development  in    hybrids    of  Oenothera  lata  x  O. 

lamarckiana  and  its  relation  to  mutation.     Bot.  Gaz.,  vol.  43. 

1908  A  study  or  reduction  in  Oenothera  rubrinervis.     Ibid.,  vol.  46. 

1909  The  behavior  of  the  chromosomes  in  Oenothera  lata  x  O.  gigas. 
Ibid.,  vol.   48. 

1911     The  mode  of  chromosome  reduction.     Ibid.,  vol.  51. 
GEERTS,  J.  M.     1909    Beitrage  zur  Kenntniss  der  Cytologie  und  der  partiellen- 

Sterilitat  von  Oenothera  lamarckiana.     Rec.  Trav.  Bot.  Ne'erland., 

Bd.  5. 

1911     Cytologische  Untersuchungen  einiger  Bastarde  von  Oenothera 

gigas.     Ber.  d.  deutsch.  Bot.  Ges.,  Bd.  29,  no.  3. 
GERARD,  P.     1909    Recherches  sur  las  spermatoge'nese  chez  Stenonothrus  bigut- 

tulus.    Arch.  Biol.,  Bd.  24. 
GREGOIRE,  V.     1904    Le  r6duction  nume'rique  des  chromosomes  et  les  cineses  de 

maturation.    La  Cellule,  torn.  21. 

1905  Les  resultats  acquis  sur  les  cineses  de  maturation  dans  les  deux 
regnes.     I.  Ibid.,      Tom.  22. 

1906  La  structure  de  I'el6ment  chromosomique  au  repos  et  en  division 
dans  less  cellules  ve"g6tales.     Ibid.,  Tom.  23. 

1907  La  formation  des  gemini  he'te'rotypiques  dans  les  ve'ge'taux.     Ibid. 
Tom.  24. 

1910  Les  cineses  de  maturation  dans  les  deux  regnes.     II.     Ibid., 
Tom.  26. 

GODLEWSKI,  E.  1911  Studien  iiber  die  Entwicklungserregung,  I.  Kombination 
der  heterogenen  Befruchtung  mit  der  kiinstlichen  Parthenogenese. 
Arch.  f.  Entwicklm.,  Bd.  33,  nos.  1,  2. 

GOLDSCHMIDT,  R.  1906  Review  of  Schreiner  ('06)  in:  Zool.  Centrb.,  Bd.  13,  nos. 
19,  20. 

1908  a     Ueber  das  Verhalten  des  Chromatins  bei  der  Eireifung  und 
Befruchtung  des  Dicrocoelium  lanceatum   (Distomum  lanceolatum). 
Arch.  f.  Zellforsch.,  Bd.  1,  no.  1. 

1908  b    1st  eine  parallele  Chromosomenkonjugation  bewiesen?    Ibid., 
Bd.  1,  no.  4. 

GROSS,  J.     1904    Die  Spermatogenese  von  Syromastes  marginatus.     Zool.  Jahrb., 
Anat.  u.  Ontog.,  Bd.  20. 
1907    Die  Spermatogenese  von  Pyrrhocoris  apterus.    Ibid.,  Bd.  23. 


428  EDMUND  B.  WILSON 

GULICK,  A.     1911    Ueber  die  Geschlechtschromosomen  bei  einigen  Nematoden, 

etc.     Arch.  f.  Zellforsch.,  Bd.  6. 
GTTTHERZ,  S.     1912    Ueber  ein  bemerkenswertes  Strukturelement  (Heterochro- 

mosom?)inder  Spermiogenese  desMenschen.    Arch.Mik.  Anat.,  Bd.  79, 

no.   2. 
HAECKER,  V.     1895    Ueber  the  Selbstandigkeit  der  vaterlichen  und  mutter-lichen 

Kerbestandteile  wahrend   der  Embryonalentwicklung  von   Cyclops. 

Arch.  Mik.  Anat.,  Bd.  46. 

1902    Ueber  das  Schicksal  der  elterlichen  und  gross-elterlichen  Kern- 

anteile.      Jen.  Zeitschr.,  Bd.  37. 

1907  Die    Chromosomen   als   angenommene   Vererbungstrager.     Er- 
gebn.  Fortschritt.  Zool.,  Bd.  1. 

1910    Ergebnisse  und  Ausblicke  in  der  Keimzellforschung.    Zeitschr. 

f.  indukt.  Abst.  u.  Verebungslehre.     Bd.  3. 
HERBST,  C.    1909   Die  cytologischenGrundlagen  der  Verschiebungder  Vererbungs- 

richtung  nach  der  miitterlichen  Seite.    Arch.  f.  Entwicklm.     Bd.  27. 

no.  2. 
JANSSENS,  F.  A.     1901    La  spermatog6nese  chez  les  tritons.     La  Cellule,  torn.  19. 

1905    Evolution desAuxocytes males duBatracosepsattenuatus.    Ibid., 

torn.  22. 

1909    La   th6orie  de  la  chiafmatypie.    Nouvelle   interpretation   des 

cineses  de  maturation.     Ibid.,  torn.  25. 
JANSSENS  ET  DTJMEZ    1903   L'el6ment  nucleinien  pendant  les  cineses  de  maturation 

des  spermatocytes  chez  Batracoseps  attenuatus  et  Plethodon  cinereus. 

Ibid.,  torn.  20. 
JANSSENS  ET  WILLEMS    1908    Spermatoge'nese  dans  les  batraciens.     IV,  Le  sper- 

matog^nese  dans  1'Alytes  obstetricus.    Ibid.,  torn.  25. 
KING,  H.  D.     1907    The  spermatogenesis  of  Bufo  lentiginosus.    Am.  Jour.  Anat., 

vol.  7. 

1908  The  oogenesis  of  Bufo  lentiginosus.    Jour.  Morph.,  vol.  19. 
KOSSEL,  A.     1912    The  proteins.    Herter  Lecture,  Johns  Hopkins,  Oct.,  1911. 

Johns  Hopkins  Hospital  Bull.,  March,  1912. 

KRIMMEL,  O.  1910  Chromosomenverhaltnisse  in  generativen  und  somatischen 
»  Mitosen  bei  Diaptomus  coeruleus,  etc.  Zool.  Anz.,  Bd.  35. 

LEE,  A.  BOLLES  1911  Le  r6duction  numerique  et  la  conjugasion  des  chromo- 
somes chez  1'escargot.  La  Cellule,  torn.  27. 

LEFEVRE  AND  McGiLL  1908  The  chromosomes  of  Anasa  tristis  and  Anax  junius. 
Am.  Jour.  Anat.,  vol.  8. 

LERAT,  P.  1905  Les  phenomenes  de  maturation  dans  1'ovogenese  et  la  sperm- 
aotogenese  du  Cyclops  strenuus.  La  Cellule,  torn  22,  no.  1. 

MATSCHEK,  H.  1910  Ueber  Eireifung  und  Eiablage  bei  Copepoden.  Arch.  f. 
Zellforsch.,  Bd.  5. 

McCLUNG,  C.  E.  1899  A  peculiar  nuclear  element  in  the  male  reproductive  cells 
of  insects.  Zool.  Bull.,  vol.  2. 

1901  Notes  on  the  Accessory  Chromosome.    Anat.  Anz.,  vol.  20. 
1902 a    The   Accessory  Chromosome — Sex  Determinant?    Biol.  Bull., 
III. 

1902  b    The  spermatocyte  divisions  of  the  Locustidae.    Kansas  Univ. 
Sci.  Bull.,  14,  p.  8. 


STUDIES  ON  CHROMOSOMES  429 

MEVES,  F.     1907    Die  Spermatocytenteilungen  bei  der  Honigbiene  nebst  Bemerk- 
ungen  tiber  Chromatinreduktion.    Arch.  Mik.  Anat.,  Bd.  70. 

1908  Es  gibt  keine  parallels  Konjugation  der  Chromosomen.    Arch, 
f.  Zellforsch.,  Bd.  1. 

1911    Chromosomenlange  bei   Salamandra,   nebst  Bemerkungen  zur 
Individualitatstheorie  der  Chromosomen.    Arch.  Mik.  Anat.,  Bd.  77. 

MONTGOMERY,  T.  H.     1901    A  study  of  the  chromosomes  of  the  germ  cells  of  Met- 
azoa.    Trans.  Am.  Phil.  Soc.,  vol.  20. 

1904    Some   observations   and   considerations   upon  the   maturation 
phenomena  of  the  germ  cells.    Biol.  Bull.,  vol.  6,  no.  3. 
1906    Chromosomes  in  the  spermatogenesis  of  the  Hemiptera  Heterop- 
tera.    Ibid.,  vol.  21. 

1911    The  spermatogenesis   of    an    Hemipteron,   Euschistus.     Jour. 
Morph.,  vol.  22,  no.  3. 

MOORE,  J.  E.  S.     1895    On  the  structural  changes  in  the  reproductive  cells  of 
Elasmobranchs.    Quart.  Journ.  Mic.  Sci.,  vol.  38. 

MOORE  AND  ROBINSON    1904    On  the  behavior  of  the  nucleolus  in  the  spermato- 
genesis of  Periplaneta  americana.     Quart.  Jouin.  Mic.  Sci.,  vol.  48. 

MORGAN,  T.  H.     1909    A  biological  and  cytological  study  of  sex  determination  in 
Phylloxerans  and  Aphids.    Jour.  Exp.  Zool.,  vol.  7,  no.  2. 
1911  a    An  attempt  to  analyze  the  constitution  of  the  chromosomes  on 
the  basis  of  sex-limited  inheritance  in  Drosophila.    Jour.  Exp.  Zool., 
vol.  11,  no.  4. 

1911  b    Random  segregation  versus  coupling  in  Mendelian  inheritance. 
Science,  n.  s.,  vol.  34,  no.  873. 

MORRILL,  C.  V.     1910    The  chromosomes  in  the  oogenesis,  fertilization  and  cleav- 
age of  Coreid  Hemiptera.    Biol.  Bull.,  vol.  19,  no.  2. 

MORSE,  M    1909    The  nuclear  components  of  the  sex-cells  in  four  species  of  cock- 
roaches.   Arch.  f.  Zellforsch.,  Bd.  3. 

MOTTIER,  D.  M.     1907    The  development  of  the  heterotype  chromosomes  in  pol- 
len mother-cells.    Ann.  Bot.,  vol.  21. 

1909  On  the  prophases  of  the  heterotypic  mitosis  in  the  embryo-sac 
mother-celr* of  Lilium.    Ibid.,  vol.  23. 

MULLER,  C.     1909    Ueber  karykinetische  Bilder  in  den  Wurzelspitzen  von  Yucca. 

Jahrb.  f.  Wiss.  Bot.,  Bd.  47,  no.  1. 
OETTINGER,  R.     1909    Zur  Kenntniss  der  Spermatogenese  bei  den  Myriopoden. 

Samenreifung  und   Samenbildung  bei   Pachyiulus   varius.     Arch.  f. 

Zellforsch.,  Bd.  3,  no.  4. 
OVERTON,  J.  B.     1905    Ueber  Reduktionsteilung  in  den  Pollenmutterzellen  einiger 

Dikotylen.    Histologische   Beitrage   zur  Vererbungsfrage.    Jahrb.    f. 

Wiss.  Bot.,  Bd.  42. 

1909    On  the  organization  of  the  nuclei  in  the  pollen  mother-cells  of 

certain  plants,  with  especial  reference  to  the  permanence  of  the  chromo- 
somes.   Ann.  Bot.,  vol.  23. 
PAULMIER,  F.  C.     1898    Chromatin  reduction  in  the  Hemiptera.    Anat.  Anz., 

Bd.  14. 

1899    The  spermatogenesis  of  Anasa  tristis.    Jour.  Morph.,  vol.  15, 

Supplement. 


430  EDMUND  B.  WILSON 

PAYNE,  F.     1909    Some  new  types  of  chromosome  distribution  and  their  relation 

to  sex.    Biol.  Bull.,  vol.  16. 
PINNEY,  E.     1908    Organization  of  the  chromosomes  in  Phrynotettix  magnus. 

Kansas  Univ.  Sci.  Bull.,  vol.  4,  no.  14. 
POPOPP,  M.     1908    Ueber  das  Vorhandensein  von  Tetraden-Chromosomen  in 

den  Leberzellen  von  Paludina  vivipara.     Biol.  Centrb.,  Bd.  28,  no.  17. 
ROBERTSON,  W.  R.  B.     1908    The  chromosome  complex  of  Syrbula.    Kansas  Univ. 

Bull.,  vol.  4,  no.  13. 
ROSENBERG,  O.     1904    Ueber  die  TetradenteilungeinesDrosera-Bastardes.     Ber. 

d.  deutsch.  bot.  Ges.,  Bd.  22. 

1909    Cytologische  und  morphologische  Studien  an  Drosera  longifolia 

x  rotundifolia.    Kungl.   Svenska  Vetenskapsakad.    Handl.,   Bd.   43, 

no.   15. 
SARGANT,  E.     1897    The  formation  of  the  sexual  nuclei  in  Liliummartagon.    Ann. 

Bot.,  vol.  9. 
SCHILLER,  J.     1909    Ueber  kiinstliche  Erzeugung  "primitiver"  Kernteilungsfor- 

men  bei  Cyclops.    Arch.  f.  Entwicklm.,  Bd.  27,  no.  4. 
SCHNEIDER,  K.  C.     1910    Histologische  Mitteilungen.     III.  Chromosomengenese. 

Festschrift  R.  Hertwig.     Bd.  1. 
SCHREINER,  A.  UND  K.  E.     1906  a     Neue   Studien  liber  Chromatinreifung  der 

Geschlechtszellen.     I.    Die    Reifung,    der    mannlichen    Geschlechts- 

zellen  von  Tomopteris  onisciformis.     Arch.  Biol.,  Bd.  22. 

1906  b    II.  Reifung  der  mannlichen  Geschlechtszellen  von  Salaman- 
dra  maculosa,  Spinax  niger  und  Myxine  glutinosa.     Ibid.,  Bd.  22. 
1908    Gibt  es  eine  parallelle  Konjugation  der  Chromosomen?    Erwid- 
erung  an  die  Herren  Fick,  Goldschmidt  und  Meves.    Videnskbs-Sel- 
skab.     Skrifter.     I.  Math.-Naturv.  Klasse,  No.  4. 

STEVENS,  N.  M.  1906  Studies  on  spermatogenesis.  II.  A  comparative  study 
of  the  heterochromosomes  in  certain  species  of  Coleoptera,  Hemiptera 
and  Lepidoptera,  with  especial  reference  to  sex  determination.  Car- 
negie Institution  Pub.  no.  36. 

1908  A  study  of  the  germ  cells  of  certain  Diptera,  with  reference  to 
the  heterochromosomes  and  the  phenomena  of  Stynapsis.  Journ.  Exp. 
Zool.,  vol.  5,  no.  3. 

STOMPS,  T.  J.  1911  Kernteilung  und  Synapsis  bei  Spinacia  oleracea.  Biol. 
Centralb.,  Bd.  21,  nos.  9-10. 

STRASBURGER,  E.  1904  Ueber  Reduktionstheilung.  Sitzungsber.  k.  k.  Preuss. 
Akad.  Wiss.,  Bd.  18. 

1905  Typische  und  allotypische  Kernteilung.  Jahrb.  f.  Wiss.  Bot., 
Bd.  41. 

1907  Ueber  die  Individualitat  der  Chromosomen  und  die  Propf-hybri- 
den-Frage.     Jahrb.  f.  Wiss.  Bot.,  Bd.  44. 

1908  Chromosomenzahlen,    Plasmastrukturne,    Verebungstrager   und 
Reduktionsteilung.     Ibid.,  Bd.  45. 

1909  Zeitpunkt  der  Bestimmung  des  Geschlechts,  Apogamie,   Par- 
thenogenese  und  Reduktionsteilung.     Jena,  G.  Fischer. 

1910  Chromosomenzahl.     Ibid. 


STUDIES  ON  CHROMOSOMES  431 

SUTTON,    W.    S.     1900    The   spermatogonial   divisions   in   Brachystola   magna. 

Kansas  Univ.  Quart.,  vol.  9. 

1902    On  the  morphology  of   the   chromosome-group  in  Brachystola 

magna.     Biol.  Bull.,  vol.  4,  no.  1. 

SYKES,  M.  G.     1909    On  the  nuclei  of  some  unisexual  plants.    Ann.  Bot.,  vol.  23. 
VEJDOVSKY,  F    1907    Xeue  Untersuchungen  uber  die  Reifung  und  Befruchtung. 

Konigl.  Bohmische  Ges.  d.  VViss.,  Prag. 

DE  VRIES,  H.     1903    Befruchtung  und  Bastardierung,  Leipzig. 
WINIWARTER   ET   SAiNTMONT    1909    Nouvelles   recherches   sur   I'ovog6nese   et 

1'organogenese  de  1'ovaires  des  mammiferes  (chat).     IV.     Ovog£nese 

de  la  zone  cortical  primitive.    Arch.  Biol.  torn.  25. 
WILSON,  E.  B.     1905  a    The  chromosomes  in  relation  to  the  determination  of  sex 

in  insects.     Science,  vol.  22.  p.  564. 

1905  b    Studies  on  chromosomes.     I.  The  behavior  of  the  idiochromo- 
somes  in  Hemiptera.     Jour.  Exp.  Zool.,  vol.  2,  no.  3. 

19C5  c  Studies,  II.  The  paired  microchromosomes,  idiochromosomes 
and  heterotropic  chromosomes  in  Hemiptera.  Ibid.,  vol.  2,  no.  4. 

1906  Studies,  III.    The  sexual  differences  of  the  chromosome-groups 
in  Hemiptera,  with  some  considerations  on  the  determination  and  hered- 
ity of  sex.     Ibid.,  vol.  3,  no.  1. 

1909  a  Studies,  IV.  The  'accessory'  chromosome  in  Syromastes  and 
Pyrrhocoris,  with  a  comparative  review  of  the  types  of  sexual  differences 
of  the  chromosome-groups.  Ibid.,  vol.  6,  no.  1. 

1909  b  The  female  chromosome-groups  in  Syromastes  and  Pyrrhocoris. 
Biol.  Bull.,  vol.  16,  no.  4. 

1909  c     Photographic  illustrations  of  the  morphological  and  physio- 
logical individuality  of  the  chromosomes  in  Hemiptera   (Abstract). 
Proc.  Seventh  Internat.  Zool.  Congr.,  Boston,  August,  1907. 
1911  a    Studies,  VII.     A  review  of  the  chromosomes  of  Nezara,  with 
some  more  general  considerations.     Jour.  Morph.,  vol.  22,  no.  1. 

1911  b    The  sex  chromosomes.     Arch.  Mik.  Anat.,  Bd.  77. 

1912  Some  aspects  of  cytology  in  relation  to  the  studv  of  genetics. 
Am.  Naturalist,  vol.  46,  February. 


PLATE  1" 

EXPLANATION   OP  FIGURES 

All  of  the  figures  are  from  Oncopeltus  fasciatus  excepting  7  and  8,  which  are  from 
Lygaeus  bicrucis.    Enlargement  2250  diameters. 
1-3    Spermatogonial  metaphases.    Oncopeltus. 
4-5    Metaphases  from  ovarian  cells. 

6  Spermatogonial  metaphase.    Lygaeus. 

7  Metaphase  of  first  spermatocyte-di vision.    Lygaeus. 

8-13    Metaphases  of  first  spermatocyte-division  in  polar  view.    Oncopeltus. 
14-17    Metaphases  of  same  in  lateral  view,  showing  the  sex-chromosomes  near 
the  center. 

18  Early  anaphase  of  same  division,  showing  all  the  chromosomes.    The  two 
at  the  right  and  the  one  at  the  left  have  been  drawn  displaced  so  as  not  to  confuse 
the  central  group. 

19  Later  anaphase,  showing  approach  of  the  sex-chromosomes. 

20  •  Slightly  later  stage,  showing  the  sex-chromosomes  in  contact  at  each  end 
of  the  spindle. 

21  Slightly  earlier  anaphase,  from  a  smear-preparation,  showing  all  the  chro- 
mosomes.   The  sex-chromosomes  already  in  conjugation  at  each  pole. 


20  All  of  the  figures  of  plates  1  to  7  have  been  drawn  as  far  as  possible  with  the 
camera  lucida,  though  of  course  at  so  great  an  enlargement  it  has  been  necessary 
to  finish  much  of  the  finer  detail  freehand.  Many  of  them  are  the  work  of  Miss 
Mabel  L.  Hedge,  to  whose  skill  and  painstaking  accuracy  especial  acknowledg- 
ment is  due.  In  many  cases  (as  indicated  in  brackets)  the  same  objects  are  shown 
by  photographs  on  plates  8  to  9.  It  should  be  added  that  some  of  the  finer  details 
appear  less  clearly  in  the  reproductions  of  these  photographs  than  in  the  original 
negatives. 

432 


STUDIES    ON    CHROMOSOMES 

EDMUND    II.    WILSON 


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PLATE  2 

EXPLANATION    OF   FIGURES 

Onco  peltus,  excepting  28,  29,  45,  46,  which  are  from  Lygaeus.  Enlargement  2250 
diameters. 

22-23  Final  anaphases  of  the  first  division.  The  sex-chromosomes  not  visible 
in  these  views. 

24-25  Polar  views  of  two  daughter-chromosome  plates  from  the  same  stage  as 
the  foregoing  (from  different  spindles)  showing  the  X F-bivalent  near  the  center 
(no.  712). 

26-27    Similar  views  of  sister-groups  from  the  same  spindle  (no.  760). 

28-29    Sister-groups  from  the  same  spindle.     Lygaeus. 

30-32  Interkineses,  showing  the  chromosome-groups,  the  first  in  side-views, 
the  others  in  face-view  showing  all  the  chromosomes  (no.  760). 

33     Prophase  of  second  division. 

34-35    Second  division  metaphase  in  polar  view. 

36-44  The  same  in  side-view,  the  sex-chromosomes  seen  separating  in  several 
of  the  figures  (36-41,  equal  type,  no.  712;  42-44,  unequal  type,  no.  760). 

45-46  (photo.  6)  Second  division  metaphases  of  Lygaeus,  lying  side  by  side 
in  the  same  section,  one  in  polar  view,  one  from  the  side. 


434 


STUDIES   ON   CHROMOSOMES 

ED.ML  Nl>     U.  WILSON 


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THE    JOUKNAI.    OK     KXPERIMENTAL    ZOOLOGY,     VOL.    13,     NO.    3. 


Wilson,    Del. 


435 


PLATE  3 

EXPLANATION   OF  FIGURES 

Oncopeltus,  from  sections  excepting  65,  which  is  from  a  smear-preparation. 
Enlargement  2250  diameters. 

47-48  Spermatogonial  telophases.  It  is  uncertain  whether  this  is  the  last  divi- 
sion or  an  earlier  one. 

49    Later  spermatogonial  telophase.     Probably  Stage  a. 

50-51  Stage  b,  showing  the  massive  chromatic  bodies,  the  sex-chromosomes 
readily  distinguishable. 

52-55    Stages  b-c,  showing  the  process  of  unravelling. 

56-59    Stage  d.     Leptotene-nuclei. 

60  Transition  to  the  synizesis.     Synaptic  period. 

61  Stage  e.     Synizesis,  from  a  very  clear  specimen. 

62  Transition  to  the  following  stage. 

63  Stage  /.     Pachytene-nucleus.     The  threads  apparently  undivided. 

64  Stage  /.     Diplotene-nucleus. 

65  (photo.  10).     Stage/.     Diplotene-nucleus,  from  a  smear-preparation. 
66-67    Stage  g.    The  confused  stage,  showing  plasmasome  and  both  chromo- 

some-nucleoli  (sex-chromosomes).     Plasmasome  at  its  maximum  size. 


436 


srrniKs   ON    CHROMOSOMES 

KDMl'ND    U.    WILSON 


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THE  JOURNAL    OK    EXPERIMENTAL    ZOOLOGY,    VOL.    13,    NO.    3 


Hecljje,  Del. 


437 


PLATE  4 

EXPLANATION   OF   FIGURES 

Lygaeus  bicrucis  (68-73,  83,  84),  Largus  cinctus  (74-82),  Anax  junius  (85-87), 
Achurum  (88-92).  Enlargement  2250,  excepting  the  figures  of  Achurum,  which 
are  enlarged  1500  diameters. 

68-70  Stage  a.  Earlier  and  later  final  spermatogonial  telophases,  from  the 
same  cyst.  Lygaeus. 

71-72    Stage  b.     Lygaeus. 

73  a-73  b    Stage  d.    Leptotene-nuclei.    Lygaeus. 

74-75    Spermatogonial  telophases,  from  the  same  cyst.     Largus. 

76-78    Stage  c.     Largus. 

79-80    Stage  d.     Leptotene-nuclei.     Largus. 

81-82    Stage  /.     Diplotene-nuclei.     Largus. 

83-84    The  same.     Lygaeus. 

85-87    Stages  b-c.    Anax. 

88-92    Stages  b-c.    Achurum. 


438 


STUDIES    ON     CHROMOSOMES 

EDMUND    It.    WILSON 


PLATE  4 


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IOUKNAI.  OK  EXI-KKIMKNTAI.  ZOOLOGY t  VOL.  13,  NO.  3. 


Hedge,  Del. 


439 


PLATE  5 

EXPLANATION   OF    FIGURES 

Phrynotettix  (93-96),  Lygaeus  (97-100),  Largus  (101-104),  Orieopeltus  (105- 
108).  From  sections,  excepting  107,  108,  which  are  from  smear-preparations. 
The  figures  of  Phrynotettix  enlarged  1500  diameters,  the  others  2250  diameters. 

93-95  Spermatogonial  prophases  of  Phrynotettix.  Fig.  93  is  an  early  stage, 
showing  massive  polarized  bodies.  The  other  figures  show  the  uncoiling  of  the 
spireme-threads  from  these  bodies,  94  in  side  view  (photo.  31),  95  as  viewed  from 
the  pole  (photo.  30).  Fig.  96  shows  two  successive  stages  lying  side  by  side  in  the 
same  cyst  (photo.  32). 

97    The  confused  stage  (Stage  g)  in  Lygaeus. 

98-99    Early  prophases  (Stage  h)  from  Lygaeus. 

100  a-h  Isolated  chromosome-nucleoli,  from  Stages/  and  g  in  Lygaeus.  showing 
various  forms  of  the  sex-chromosomes  assumed  during  the  growth-period. 

101-103  Largus  cinctus.  Nuclei  transitional  from  Stage/  (diplotene)  to  Stage 
g  (confused  period). 

104    Nucleus  of  the  confused  period.     Largus  cinctus. 

105-106    Early  prophase-nuclei,  Oncopeltus  (early  Stage  h). 

107  (photo.  17)     Slightly  later  prophase-nucleus  of  Oncopeltus,  showing  early 
bivalents. 

108  (photo.  18)     Later  prophase  of  the  same,  early  Stage  i. 


440 


STUDIES     ON     CIIKOMOSOMKS 

EDMUND    B.    WILhON 


PLATE  5 


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TUB  JOURNAL   OK    EXl'EKIM  ENTAI.    ZOOLOGY,    VOL.    13,    NO.    3. 


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441 


PLATE  6 

EXPLANATION    OF   FIGURES 

From  smear-preparations  of  Oncopeltus  (109-114)  and  Protenor  (115-120); 
Enlargement  2250  diameters.  (X  designates  the  JT-chromosome,  B  the  large 
bivalent  in  Protenor,  m,  m,  the  m-chromosomes  in  the  latter  form. 

109-114  Middle  and  late  prophases  of  the  first  spermatocyte-division,  showing 
various  forms  of  the  bivalents  during  their  condensation.  In  the  earlier  figures 
the  X-  and  F-chromosomes  are  short,  longitudinally  split  rods  (109-111);  in  the  later 
ones  they  are  shortening  to  a  dumb-bell  form  (112-114).  Two  of  the  same  nuclei 
are  shown  in  photos.  20,  21. 

115-117  Early  prophases  (Stage  h),  the  bivalents  just  emerging  from  the  con- 
fused stage.  The  TO-chromosomes  are  but  vaguely  distinguishable. 

118-119  Late  Stage  h,  showing  all  the  chromosomes,  the  bivalents  still  much 
diffused. 

120    (photo.  40)    Nucleus  from  Stage  i. 


442 


STUDIES    ON    CHROMOSOMES 

EDMUND    H.    WILSON 


PLATE  6 


109 


110 


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IHK  JOURNAL    OK    EXfEKIM  ENTAI.    ZOOLUGYt    VOL.    13,    NO.   3. 


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443 


PLATE  7 

EXPLANATION   OF    FIGURES 

From  smear-preparations  of  Protenor;  2250  diameters;  (lettering  as  in  the  pre- 
ceding plate). 

121-127  Middle  prophases  (Stage  i)  showing  all  the  chromosomes.  The  m- 
chromosomes,  now  condensed,  are  separate  in  all  but  124.  In  126  the  large  biva- 
lent is  a  straight  rod;  in  127  it  is  such  a  rod  bent  at  the  middle  point  to  form  a  V 
(here  seen  edgewise  so  as  not  to  show  the  longitudinal  cleft.  Some  of  the  same 
nuclei  are  shown  in  photos.  41;  44,  47. 

128  (photo.  43)  Abnormal  nucleus  in  which  the  large  bivalent  is  represented 
by  a  pair  of  separate  univalents  (B,  B)  that  have  failed  to  unite  in  synapsis. 

129-131  Late  prophases  (Stage  j).  In  131  the  chromosomes  are  ready  to  enter 
the  inetaphase-plate;  (fig.  131  also  in  photo.  51). 

132-134  Early  metaphases.  In  132  one  of  the  small  bivalents  and  the  m-chro- 
mosomes  appear  abnormally  large,  owing  to  flattening.  In  134  the  ring  tetrad 
to  the  left  has  been  slightly  displaced  in  order  to  show  it  more  clearly. 


444 


STt'DlKS    OX     CHROMOSOMES 

EDMUND    H.    WILSON 


PLATE  7 


130 


131 


134 


THE  JOUKNAI.    OK    EXfEKIM  KNTAI.    ZOOLOGY,    VOL.    13,    NO.    3. 


Hedge,  Del. 


PLATE  8 

EXPLANATION   OF   FIGURES 

From  photographs  by  the  author.  Enlargement  a  little  less  than  1250  diameters. 
Oncopeltus  (1-5,  7-11,  16-24),  Lygaeus  bicrucis  (6,  12-15,  25),  Largus  cinctus  (26, 
27).  Many  figures  of  the  same  preparations,  from  drawings,  are  reproduced  in  the 
preceding  plates,  as  indicated  in  brackets.  Photos.  1-15,  26,27,  from  sections, 
the  others  from  smear-preparations. 

1, 2    Spermatogonial  metaphases.    Oncopeltus. 

3  First  division  metaphase. 

4  Second  division  metaphase. 

5  First  division  metaphase  in  side  view,  showing  the  sex-chromosomes  in  the 
center. 

6  (Figs.  45,  46)     Second  division  metaphases,  one  in  polar  view,  one  in  side 
view,  showing  the  initial  separation  of  X  and  Y.     Lygaeus  bicrucis. 

7-8    Stage  d.     Leptotene-nuclei  of  Oncopeltus. 

9  Pachytene-nuclei,  just  emerging  from  the  synizesis.     Oncopeltus. 

10  (Fig.  65).     Stage/.     Early  diplotene-nucleus.     From  a  smear. 

11  Stage  g.     Confused  stage,  showing  plasmasome  and  both  sex-chromosomes. 

12  Stage  e.     Synizesis,  Lygaeus,  from  much  extracted  preparation,  showing  the 
X-  and  F-chromosomes  united. 

13  Above,  the  X-  and  F-chromosomes  of  Lygaeus  in  Stage  /,  attached  in  one 
case  end  to  end,  in  the  other  side  by  side.     Below,  the  same  from  early  Stage  g, 
showing  also  the  plasmasome. 

14-15  Nuclei  of  Stage  g,  Lygaeus,  showing  the  longitudinally  divided  X-chro- 
mosome,  and  (at  the  left)  the  plasmasome. 

16    Nucleus  of  the  confused  period  (Stage  g).    Oncopeltus. 

17-24  Early,  middle  and  late  prophases  (Stages  h-j)  from  smear-preparations 
of  Oncopeltus.  Photo.  17  (fig.  107),  18  (fig.  108),  20  (fig.  109),  21  (fig.  111).  The 
sex-chromosomes  distinguishable  in  each  case. 

25  Late  prophase-nucleus  of  Lygaeus  (Stage  i-j),  the  X-  and  F-chromosomes 
readily  distinguishable  above  towards  the  left. 

26-27  Stage  b-c,  in  Largus  cinctus.  The  uncoiling  of  spiral  leptotene-threads 
is  clearly  visible  in  the  negative  of  photo.  27. 


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PLATE  9 

EXPLANATION    OF  FIGURES 

From  photographs  by  the  author.  Enlargement  as  in  the  preceding  plate. 
Photos.  28-38  from  sections  (28-32  from  McClung's  preparations),  39-51  from 
smear-preparations.  Photo.  28,  Achurum,  29-32  Phrynotettix,  33,  34  Largus  cinc- 
tus,  35-51  Protenor  belfragei. 

28  Stage  c  in  Achurum.     The  unravelling  threads  clearly  shown  in  the  negative. 

29  Early  spermatogonial  prophase  of  Phrynotettix,  showing  the  massive  chro- 
matin-bodies  just  before  the  spiral  thread  is  evident. 

30  (Fig.  95).     At  the  left,  polar  view  of  the  coiled  threads  during  the  early 
uncoiling,  spermatogonial  prophase. 

31  (Fig.  94).    The  same  stage  (from  a  nucleus  immediately  adjoining  in  the 
same  section)  seen  in  side-view. 

32  (Fig.  96).     Two  adjoining  nuclei,  showing  at  the  left  an  earlier,  and  at  the 
right  a  later  stage  of  the  uncoiling  of  the  spireme-threads.     The  drawings  of 
these  and  the  preceding  photo,  show  threads  at  other  levels  as  well. 

33  Spermatogonial  metaphase  of  Largus  cinctus,  11  chromosomes,  including 
one  large  pair.     The  X-chromosome  is  one  of  the  smaller  ones,  and  can  not  be  dis- 
tinguished by  the  eye. 

34  Metaphase  of  diploid  group  of  the  female  of  the  same  species,  12  chromo- 
somes. 

35  (Fig.  1  e,  Wilson,  '06).     Spermatogonial  metaphase  of  Protenor;  13  chromo- 
somes. 

36  The  same.     In  both  these  photos,  the  large  ^"-chromosome  and  the  large 
pair  of  autosomes  are  readily  distinguishable. 

37-38  Diploid  chromosome-groups  of  the  female  Protenor,  showing  the  -X"-pair 
and  the  large  pair  of  autosomes;  14  chromosomes. 

39  Protenor.  Above,  a  nucleus  of  the  confused  stage  (g)  showing  the  elongate 
J£-chromosome.  Below  are  three  final  anaphases  of  the  second  division,  showing 
the  passage  of  the  undivided  ^-chromosome  to  one  pole. 

40-51  Prophase-nuclei  of  Protenor.  Photo.  40  (fig.  120),  41  (fig.  122)  43  (fig. 
128),  44  (fig.  121),  46  (fig.  129),  47  (fig.  127),  51  (fig.  131). 


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THE  JOURNAL  OF  EXPERIMENTAL  ZOOLOGY, 

VOLUME  13,  NUMBER  3,  OCTOBER,  1912 


CONTENTS 

Edmund  B.  Wilson 

Studies  on  chromosomes.  VIII.  Observations  on  the  matur- 
ation-phenomena in  certain  Hemiptera  and  other  forms, 
with  considerations  on  synapsis  and  reduction.  vrom  the 
Department  of  Zoology,  Columbia  University.  N  ne  plates  345 

Wayland  M.  Chester 

Wound  closure  and  polarity  in  the  tentacle  of  Metridium 
marginatum.  From  The  Museum  of  Comparative  Zoology, 
Harvard  College.  Eight  figures 451 

Max  Morse 

Artificial  parthenogenesis  and  hybridization  in  the  eggs  of  cer- 
tain invertebrates.  From  Trinitv  College .  .  471 


THE    WAVERLY    PRESS 

BALTIMORE,    U.    S.    A. 


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